diff --git a/pkg/_fe_analyzer_shared/lib/src/util/libraries_specification.dart b/pkg/_fe_analyzer_shared/lib/src/util/libraries_specification.dart
index bcbe4856162..7bc929b5fd1 100644
--- a/pkg/_fe_analyzer_shared/lib/src/util/libraries_specification.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/util/libraries_specification.dart
@@ -76,6 +76,9 @@
 ///     report that such library is still not supported in conditional imports
 ///     and const `fromEnvironment` expressions.
 ///
+///     Internal libraries are never supported through conditional imports and
+///     const `fromEnvironment` expressions.
+///
 ///
 /// Note: we currently have several different files that need to be updated
 /// when changing libraries, sources, and patch files:
@@ -270,8 +273,10 @@ class LibrariesSpecification {
         if (supported is! bool) {
           _reportError(messageSupportedIsNotABool(supported));
         }
-        libraries[libraryName] =
-            new LibraryInfo(libraryName, uri, patches, isSupported: supported);
+        libraries[libraryName] = new LibraryInfo(libraryName, uri, patches,
+            // Internal libraries are never supported through conditional
+            // imports and const `fromEnvironment` expressions.
+            isSupported: supported && !libraryName.startsWith('_'));
       });
       currentTargets.remove(targetName);
       return targets[targetName] =
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 2b0ad7dc68d..ed1087c7893 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -5,10 +5,6 @@
 library leg_apiimpl;
 
 import 'dart:async';
-import 'dart:convert' show utf8;
-
-import 'package:front_end/src/api_unstable/dart2js.dart'
-    show getSupportedLibraryNames;
 
 import '../compiler_new.dart' as api;
 import 'common/metrics.dart' show Metrics, Metric;
@@ -17,7 +13,6 @@ import 'common.dart';
 import 'compiler.dart';
 import 'diagnostics/messages.dart' show Message;
 import 'environment.dart';
-import 'io/source_file.dart';
 import 'options.dart' show CompilerOptions;
 
 /// Implements the [Compiler] using a [api.CompilerInput] for supplying the
@@ -58,61 +53,22 @@ class CompilerImpl extends Compiler {
         null, null, null, null, message, api.Diagnostic.VERBOSE_INFO);
   }
 
-  Future setupSdk() {
-    var future = Future.value(null);
-    _Environment env = environment;
-    if (env.supportedLibraries == null) {
-      future = future.then((_) {
-        Uri specificationUri = options.librariesSpecificationUri;
-
-        Future<String> readJson(Uri uri) async {
-          api.Input spec = await provider.readFromUri(specificationUri);
-          String json = null;
-          // TODO(sigmund): simplify this, we have some API inconsistencies when
-          // our internal input adds a terminating zero.
-          if (spec is SourceFile) {
-            json = spec.slowText();
-          } else if (spec is Binary) {
-            json = utf8.decode(spec.data);
-          }
-          return json;
-        }
-
-        // TODO(sigmund): would be nice to front-load some of the CFE option
-        // processing and parse this .json file only once.
-        return getSupportedLibraryNames(specificationUri,
-                options.compileForServer ? "dart2js_server" : "dart2js",
-                readJson: readJson)
-            .then((libraries) {
-          env.supportedLibraries = libraries.toSet();
-        });
-      });
-    }
-    // TODO(johnniwinther): This does not apply anymore.
-    // The incremental compiler sets up the sdk before run.
-    // Therefore this will be called a second time.
-    return future;
-  }
-
   @override
   Future<bool> run() {
     Duration setupDuration = measurer.elapsedWallClock;
-    return selfTask.measureSubtask("impl.run", () {
-      return setupSdk().then((_) {
-        return super.run();
-      }).then((bool success) {
-        if (options.verbose) {
-          StringBuffer timings = StringBuffer();
-          computeTimings(setupDuration, timings);
-          logVerbose('$timings');
-        }
-        if (options.reportPrimaryMetrics || options.reportSecondaryMetrics) {
-          StringBuffer metrics = StringBuffer();
-          collectMetrics(metrics);
-          logInfo('$metrics');
-        }
-        return success;
-      });
+    return selfTask.measureSubtask("impl.run", () async {
+      bool success = await super.run();
+      if (options.verbose) {
+        StringBuffer timings = StringBuffer();
+        computeTimings(setupDuration, timings);
+        logVerbose('$timings');
+      }
+      if (options.reportPrimaryMetrics || options.reportSecondaryMetrics) {
+        StringBuffer metrics = StringBuffer();
+        collectMetrics(metrics);
+        logInfo('$metrics');
+      }
+      return success;
     });
   }
 
@@ -239,50 +195,18 @@ class CompilerImpl extends Compiler {
 class _Environment implements Environment {
   final Map<String, String> definitions;
   Map<String, String> _completeMap;
-  Set<String> supportedLibraries;
 
   _Environment(this.definitions);
 
-  @override
-  String valueOf(String name) {
-    if (_completeMap != null) return _completeMap[name];
-    var result = definitions[name];
-    if (result != null || definitions.containsKey(name)) return result;
-    if (!name.startsWith(_dartLibraryEnvironmentPrefix)) return null;
-
-    String libraryName = name.substring(_dartLibraryEnvironmentPrefix.length);
-
-    // Private libraries are not exposed to the users.
-    if (libraryName.startsWith("_")) return null;
-    if (supportedLibraries.contains(libraryName)) return "true";
-    return null;
-  }
-
   @override
   Map<String, String> toMap() {
     if (_completeMap == null) {
       _completeMap = Map<String, String>.from(definitions);
-      for (String libraryName in supportedLibraries) {
-        if (!libraryName.startsWith("_")) {
-          String key = '${_dartLibraryEnvironmentPrefix}${libraryName}';
-          if (!definitions.containsKey(key)) {
-            _completeMap[key] = "true";
-          }
-        }
-      }
     }
     return _completeMap;
   }
 }
 
-/// For every 'dart:' library, a corresponding environment variable is set
-/// to "true". The environment variable's name is the concatenation of
-/// this prefix and the name (without the 'dart:'.
-///
-/// For example 'dart:html' has the environment variable 'dart.library.html' set
-/// to "true".
-const String _dartLibraryEnvironmentPrefix = 'dart.library.';
-
 class _TimingData {
   final String description;
   final int milliseconds;
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index dad3d72052b..35d9f525867 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1086,9 +1086,6 @@ class _MapImpactCacheDeleter implements ImpactCacheDeleter {
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
-  @override
-  String valueOf(String key) => null;
-
   @override
   Map<String, String> toMap() => const {};
 }
diff --git a/pkg/compiler/lib/src/environment.dart b/pkg/compiler/lib/src/environment.dart
index 67db9ac1c38..efc2bb0a3b0 100644
--- a/pkg/compiler/lib/src/environment.dart
+++ b/pkg/compiler/lib/src/environment.dart
@@ -8,12 +8,6 @@
 /// conditional imports, and from `const String.fromEnvironment` and
 /// other similar constructors.
 abstract class Environment {
-  /// Return the string value of the given key.
-  ///
-  /// Note that `bool.fromEnvironment` and `int.fromEnvironment` are also
-  /// implemented in terms of `String.fromEnvironment`.
-  String valueOf(String key);
-
   /// Returns the full environment as map.
   Map<String, String> toMap();
 }
diff --git a/pkg/compiler/lib/src/ir/constants.dart b/pkg/compiler/lib/src/ir/constants.dart
index 6994046cb11..d95ffb21ddc 100644
--- a/pkg/compiler/lib/src/ir/constants.dart
+++ b/pkg/compiler/lib/src/ir/constants.dart
@@ -2,11 +2,11 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:front_end/src/api_prototype/constant_evaluator.dart' as ir;
+import 'package:front_end/src/api_unstable/dart2js.dart' as ir;
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/src/printer.dart' as ir;
 import 'package:kernel/type_environment.dart' as ir;
-import 'package:front_end/src/api_prototype/constant_evaluator.dart' as ir;
-import 'package:front_end/src/api_unstable/dart2js.dart' as ir;
 
 import '../kernel/dart2js_target.dart';
 
@@ -18,7 +18,7 @@ class Dart2jsConstantEvaluator extends ir.ConstantEvaluator {
 
   bool requiresConstant;
 
-  Dart2jsConstantEvaluator(
+  Dart2jsConstantEvaluator(ir.Component component,
       ir.TypeEnvironment typeEnvironment, ReportErrorFunction reportError,
       {Map<String, String> environment = const {},
       bool supportReevaluationForTesting = false,
@@ -26,7 +26,9 @@ class Dart2jsConstantEvaluator extends ir.ConstantEvaluator {
       : _supportReevaluationForTesting = supportReevaluationForTesting,
         assert(evaluationMode != null),
         super(
+            const Dart2jsDartLibrarySupport(),
             const Dart2jsConstantsBackend(supportsUnevaluatedConstants: false),
+            component,
             environment,
             typeEnvironment,
             ErrorReporter(reportError),
diff --git a/pkg/compiler/lib/src/js_model/element_map_impl.dart b/pkg/compiler/lib/src/js_model/element_map_impl.dart
index 3bb766f2972..ae9a0a08935 100644
--- a/pkg/compiler/lib/src/js_model/element_map_impl.dart
+++ b/pkg/compiler/lib/src/js_model/element_map_impl.dart
@@ -1164,14 +1164,15 @@ class JsKernelToElementMap implements JsToElementMap, IrToElementMap {
   }
 
   Dart2jsConstantEvaluator get constantEvaluator {
-    return _constantEvaluator ??= Dart2jsConstantEvaluator(typeEnvironment,
-        (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
+    return _constantEvaluator ??=
+        Dart2jsConstantEvaluator(programEnv.mainComponent, typeEnvironment,
+            (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
       reportLocatedMessage(reporter, message, context);
     },
-        environment: _environment.toMap(),
-        evaluationMode: options.useLegacySubtyping
-            ? ir.EvaluationMode.weak
-            : ir.EvaluationMode.strong);
+            environment: _environment.toMap(),
+            evaluationMode: options.useLegacySubtyping
+                ? ir.EvaluationMode.weak
+                : ir.EvaluationMode.strong);
   }
 
   @override
diff --git a/pkg/compiler/lib/src/kernel/dart2js_target.dart b/pkg/compiler/lib/src/kernel/dart2js_target.dart
index 01f42be21c9..156e7a4f6a0 100644
--- a/pkg/compiler/lib/src/kernel/dart2js_target.dart
+++ b/pkg/compiler/lib/src/kernel/dart2js_target.dart
@@ -221,6 +221,10 @@ class Dart2jsTarget extends Target {
   @override
   ConstantsBackend get constantsBackend =>
       const Dart2jsConstantsBackend(supportsUnevaluatedConstants: true);
+
+  @override
+  DartLibrarySupport get dartLibrarySupport =>
+      const Dart2jsDartLibrarySupport();
 }
 
 // TODO(sigmund): this "extraRequiredLibraries" needs to be removed...
@@ -314,3 +318,8 @@ class Dart2jsConstantsBackend extends ConstantsBackend {
   @override
   NumberSemantics get numberSemantics => NumberSemantics.js;
 }
+
+class Dart2jsDartLibrarySupport extends CustomizedDartLibrarySupport {
+  const Dart2jsDartLibrarySupport()
+      : super(supported: const {'_dart2js_runtime_metrics'});
+}
diff --git a/pkg/compiler/lib/src/kernel/element_map_impl.dart b/pkg/compiler/lib/src/kernel/element_map_impl.dart
index f82c5eff943..b4a93afa558 100644
--- a/pkg/compiler/lib/src/kernel/element_map_impl.dart
+++ b/pkg/compiler/lib/src/kernel/element_map_impl.dart
@@ -812,14 +812,15 @@ class KernelToElementMapImpl implements KernelToElementMap, IrToElementMap {
   }
 
   Dart2jsConstantEvaluator get constantEvaluator {
-    return _constantEvaluator ??= Dart2jsConstantEvaluator(typeEnvironment,
-        (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
+    return _constantEvaluator ??=
+        Dart2jsConstantEvaluator(env.mainComponent, typeEnvironment,
+            (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
       reportLocatedMessage(reporter, message, context);
     },
-        environment: _environment.toMap(),
-        evaluationMode: options.useLegacySubtyping
-            ? ir.EvaluationMode.weak
-            : ir.EvaluationMode.strong);
+            environment: _environment.toMap(),
+            evaluationMode: options.useLegacySubtyping
+                ? ir.EvaluationMode.weak
+                : ir.EvaluationMode.strong);
   }
 
   @override
diff --git a/pkg/compiler/test/analyses/analysis_helper.dart b/pkg/compiler/test/analyses/analysis_helper.dart
index 559d972f639..b104a00d58a 100644
--- a/pkg/compiler/test/analyses/analysis_helper.dart
+++ b/pkg/compiler/test/analyses/analysis_helper.dart
@@ -86,7 +86,7 @@ class StaticTypeVisitorBase extends StaticTypeVisitor {
             classHierarchy,
             new StaticTypeCacheImpl()) {
     _constantEvaluator = new Dart2jsConstantEvaluator(
-        typeEnvironment, const ir.SimpleErrorReporter().report,
+        component, typeEnvironment, const ir.SimpleErrorReporter().report,
         evaluationMode: evaluationMode);
   }
 
diff --git a/pkg/compiler/test/end_to_end/dill_loader_test.dart b/pkg/compiler/test/end_to_end/dill_loader_test.dart
index 7bf0330040d..46626f43e18 100644
--- a/pkg/compiler/test/end_to_end/dill_loader_test.dart
+++ b/pkg/compiler/test/end_to_end/dill_loader_test.dart
@@ -43,7 +43,6 @@ main() {
         memorySourceFiles: {'main.dill': kernelBinary},
         diagnosticHandler: diagnostics,
         outputProvider: output);
-    await compiler.setupSdk();
     KernelResult result = await compiler.kernelLoader.load();
     compiler.frontendStrategy.registerLoadedLibraries(result);
 
diff --git a/pkg/compiler/test/end_to_end/library_env_test.dart b/pkg/compiler/test/end_to_end/library_env_test.dart
deleted file mode 100644
index f194f370bf1..00000000000
--- a/pkg/compiler/test/end_to_end/library_env_test.dart
+++ /dev/null
@@ -1,146 +0,0 @@
-// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// @dart = 2.7
-
-/// Check that 'dart:' libraries have their corresponding dart.library.X
-/// environment variable set.
-
-import 'dart:async';
-
-import '../helpers/memory_compiler.dart';
-import '../helpers/memory_source_file_helper.dart';
-
-import "package:async_helper/async_helper.dart";
-
-import 'package:expect/expect.dart' show Expect;
-
-import 'package:compiler/src/null_compiler_output.dart' show NullCompilerOutput;
-
-import 'package:compiler/src/options.dart' show CompilerOptions;
-
-import 'package:compiler/src/commandline_options.dart';
-
-import 'package:compiler/src/io/source_file.dart' show Binary;
-
-import 'package:compiler/compiler_new.dart'
-    show CompilerInput, CompilerDiagnostics, Input, InputKind;
-
-const String librariesJson = r'''
-{
- "dart2js": {
-   "libraries": {
-    "mock.client": {"uri": "mock1.dart"},
-    "mock.shared": {"uri": "mock3.dart"},
-    "collection": {"uri": "collection/collection.dart"},
-    "html": {"uri": "html/dart2js/html_dart2js.dart"}
-   }
- },
- "dart2js_server": {
-   "libraries": {
-    "mock.server": {"uri": "mock2.dart"},
-    "mock.shared": {"uri": "mock3.dart"},
-    "collection": {"uri": "collection/collection.dart"},
-    "io": {"uri": "io/io.dart"}
-   }
- }
-}
-''';
-
-class DummyCompilerInput implements CompilerInput {
-  const DummyCompilerInput();
-
-  @override
-  Future<Input> readFromUri(Uri uri,
-      {InputKind inputKind: InputKind.UTF8}) async {
-    if (uri.path.endsWith("libraries.json")) {
-      return new Binary(uri, librariesJson.codeUnits);
-    } else {
-      throw "should not be needed $uri";
-    }
-  }
-}
-
-class DummyCompilerDiagnostics implements CompilerDiagnostics {
-  const DummyCompilerDiagnostics();
-
-  @override
-  report(code, uri, begin, end, text, kind) {
-    throw "should not be needed";
-  }
-}
-
-class CustomCompiler extends CompilerImpl {
-  CustomCompiler(List<String> options, Map<String, String> environment)
-      : super(
-            const DummyCompilerInput(),
-            const NullCompilerOutput(),
-            const DummyCompilerDiagnostics(),
-            CompilerOptions.parse(
-                ['--platform-binaries=$sdkPlatformBinariesPath']
-                  ..addAll(options),
-                librariesSpecificationUri: sdkLibrariesSpecificationUri)
-              ..environment = environment);
-}
-
-runTest() async {
-  {
-    final compiler = new CustomCompiler([], {});
-
-    await compiler.setupSdk();
-    final lookup = compiler.environment.valueOf;
-
-    // Core libraries are always present.
-    Expect.equals("true", lookup("dart.library.collection"));
-    // Non-existing entries in the environment return 'null'.
-    Expect.isNull(lookup("not in env"));
-    // Check for client libraries (default if there are no flags to the compiler).
-    Expect.equals("true", lookup("dart.library.mock.client"));
-    Expect.equals("true", lookup("dart.library.html"));
-    // Check for shared libraries..
-    Expect.equals("true", lookup("dart.library.mock.shared"));
-    // Check server libraries are not present.
-    Expect.equals(null, lookup("dart.library.mock.server"));
-    Expect.equals(null, lookup("dart.library.io"));
-  }
-  {
-    final compiler = new CustomCompiler([Flags.serverMode], {});
-
-    await compiler.setupSdk();
-    final lookup = compiler.environment.valueOf;
-
-    // Core libraries are always present.
-    Expect.equals("true", lookup("dart.library.collection"));
-    // Non-existing entries in the environment return 'null'.
-    Expect.isNull(lookup("not in env"));
-    // Check client libraries are not present.
-    Expect.equals(null, lookup("dart.library.mock.client"));
-    Expect.equals(null, lookup("dart.library.html"));
-    // Check for shared libraries..
-    Expect.equals("true", lookup("dart.library.mock.shared"));
-    // Check for server libraries.
-    Expect.equals("true", lookup("dart.library.mock.server"));
-    Expect.equals("true", lookup("dart.library.io"));
-  }
-  {
-    // Check that user-defined env-variables win.
-    final compiler = new CustomCompiler([], {
-      'dart.library.collection': "false",
-      'dart.library.mock.client': "foo"
-    });
-
-    await compiler.setupSdk();
-    final lookup = compiler.environment.valueOf;
-
-    Expect.equals("false", lookup("dart.library.collection"));
-    Expect.equals("foo", lookup("dart.library.mock.client"));
-  }
-}
-
-main() {
-  asyncStart();
-  runTest().then((_) {
-    asyncEnd();
-  });
-}
diff --git a/pkg/compiler/test/end_to_end/modular_loader_test.dart b/pkg/compiler/test/end_to_end/modular_loader_test.dart
index 35c1066eb64..b5819f07c73 100644
--- a/pkg/compiler/test/end_to_end/modular_loader_test.dart
+++ b/pkg/compiler/test/end_to_end/modular_loader_test.dart
@@ -44,7 +44,6 @@ main() {
         memorySourceFiles: {'a.dill': aDill, 'b.dill': bDill, 'c.dill': cDill},
         diagnosticHandler: diagnostics,
         outputProvider: output);
-    await compiler.setupSdk();
     KernelResult result = await compiler.kernelLoader.load();
     compiler.frontendStrategy.registerLoadedLibraries(result);
 
diff --git a/pkg/compiler/test/model/cfe_constant_evaluation_test.dart b/pkg/compiler/test/model/cfe_constant_evaluation_test.dart
index b133fd4ca30..24c4a58f186 100644
--- a/pkg/compiler/test/model/cfe_constant_evaluation_test.dart
+++ b/pkg/compiler/test/model/cfe_constant_evaluation_test.dart
@@ -596,9 +596,9 @@ Future testData(TestData data) async {
         expectedResults
             .forEach((Map<String, String> environment, String expectedText) {
           List<String> errors = [];
-          Dart2jsConstantEvaluator evaluator =
-              new Dart2jsConstantEvaluator(elementMap.typeEnvironment,
-                  (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
+          Dart2jsConstantEvaluator evaluator = new Dart2jsConstantEvaluator(
+              elementMap.env.mainComponent, elementMap.typeEnvironment,
+              (ir.LocatedMessage message, List<ir.LocatedMessage> context) {
             // TODO(johnniwinther): Assert that `message.uri != null`. Currently
             // all unevaluated constants have no uri.
             // The actual message is a "constant errors starts here" message,
@@ -606,11 +606,11 @@ Future testData(TestData data) async {
             errors.add(context.first.code.name);
             reportLocatedMessage(elementMap.reporter, message, context);
           },
-                  environment: environment,
-                  supportReevaluationForTesting: true,
-                  evaluationMode: compiler.options.useLegacySubtyping
-                      ? ir.EvaluationMode.weak
-                      : ir.EvaluationMode.strong);
+              environment: environment,
+              supportReevaluationForTesting: true,
+              evaluationMode: compiler.options.useLegacySubtyping
+                  ? ir.EvaluationMode.weak
+                  : ir.EvaluationMode.strong);
           ir.Constant evaluatedConstant = evaluator.evaluate(
               new ir.StaticTypeContext(node, typeEnvironment), initializer);
 
diff --git a/pkg/dev_compiler/lib/src/kernel/command.dart b/pkg/dev_compiler/lib/src/kernel/command.dart
index 5b514be2ff4..9b9fd08e442 100644
--- a/pkg/dev_compiler/lib/src/kernel/command.dart
+++ b/pkg/dev_compiler/lib/src/kernel/command.dart
@@ -825,10 +825,6 @@ Map<String, String> parseAndRemoveDeclaredVariables(List<String> args) {
     }
   }
 
-  // Add platform defined variables
-  // TODO(47243) Remove when all code paths read these from the `Target`.
-  declaredVariables.addAll(sdkLibraryEnvironmentDefines);
-
   return declaredVariables;
 }
 
diff --git a/pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart b/pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart
index 6d5c4182787..215ba19c6dc 100644
--- a/pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart
+++ b/pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart
@@ -25,7 +25,6 @@ import '../../dev_compiler.dart';
 import '../compiler/js_names.dart';
 import 'asset_file_system.dart';
 import 'command.dart';
-import 'target.dart' show sdkLibraryEnvironmentDefines;
 
 /// The service that handles expression compilation requests from
 /// the debugger.
@@ -231,8 +230,6 @@ class ExpressionCompilerWorker {
       ..omitPlatform = true
       ..environmentDefines = {
         if (environmentDefines != null) ...environmentDefines,
-        // TODO(47243) Remove when all code paths read these from the `Target`.
-        ...sdkLibraryEnvironmentDefines
       }
       ..explicitExperimentalFlags = explicitExperimentalFlags
       ..onDiagnostic = _onDiagnosticHandler(errors, warnings, infos)
diff --git a/pkg/dev_compiler/lib/src/kernel/target.dart b/pkg/dev_compiler/lib/src/kernel/target.dart
index bb79eb83e96..90dc78ecd92 100644
--- a/pkg/dev_compiler/lib/src/kernel/target.dart
+++ b/pkg/dev_compiler/lib/src/kernel/target.dart
@@ -20,34 +20,6 @@ import 'package:kernel/transformations/track_widget_constructor_locations.dart';
 import 'constants.dart' show DevCompilerConstantsBackend;
 import 'kernel_helpers.dart';
 
-/// Boolean environment variables that indicate which libraries are available in
-/// dev compiler.
-// TODO(jmesserly): provide an option to compile without dart:html & friends?
-const sdkLibraryEnvironmentDefines = {
-  'dart.isVM': 'false',
-  'dart.library.async': 'true',
-  'dart.library.core': 'true',
-  'dart.library.collection': 'true',
-  'dart.library.convert': 'true',
-  // TODO(jmesserly): this is not really supported in dart4web other than
-  // `debugger()`
-  'dart.library.developer': 'true',
-  'dart.library.io': 'false',
-  'dart.library.isolate': 'false',
-  'dart.library.js': 'true',
-  'dart.library.js_util': 'true',
-  'dart.library.math': 'true',
-  'dart.library.mirrors': 'false',
-  'dart.library.typed_data': 'true',
-  'dart.library.indexed_db': 'true',
-  'dart.library.html': 'true',
-  'dart.library.html_common': 'true',
-  'dart.library.svg': 'true',
-  'dart.library.ui': 'false',
-  'dart.library.web_audio': 'true',
-  'dart.library.web_gl': 'true',
-};
-
 /// A kernel [Target] to configure the Dart Front End for dartdevc.
 class DevCompilerTarget extends Target {
   DevCompilerTarget(this.flags);
@@ -59,10 +31,6 @@ class DevCompilerTarget extends Target {
 
   Map<String, Class>? _nativeClasses;
 
-  @override
-  Map<String, String> updateEnvironmentDefines(Map<String, String> map) =>
-      map..addAll(sdkLibraryEnvironmentDefines);
-
   @override
   bool get enableSuperMixins => true;
 
diff --git a/pkg/front_end/lib/src/api_unstable/dart2js.dart b/pkg/front_end/lib/src/api_unstable/dart2js.dart
index d008fd6303d..357b21f4e51 100644
--- a/pkg/front_end/lib/src/api_unstable/dart2js.dart
+++ b/pkg/front_end/lib/src/api_unstable/dart2js.dart
@@ -12,9 +12,6 @@ import 'package:_fe_analyzer_shared/src/messages/severity.dart' show Severity;
 
 import 'package:_fe_analyzer_shared/src/scanner/scanner.dart' show StringToken;
 
-import 'package:_fe_analyzer_shared/src/util/libraries_specification.dart'
-    show LibrariesSpecification;
-
 import 'package:kernel/kernel.dart' show Component;
 
 import 'package:kernel/ast.dart' as ir;
@@ -217,28 +214,6 @@ Future<Component?> compile(
   return compilerResult?.component;
 }
 
-/// Retrieve the name of the libraries that are supported by [target] according
-/// to the libraries specification [json] file.
-///
-/// Dart2js uses these names to determine the value of library environment
-/// constants, such as `const bool.fromEnvironment("dart.library.io")`.
-// TODO(sigmund): refactor dart2js so that we can retrieve this data later in
-// the compilation pipeline. At that point we can get it from the CFE
-// results directly and completely hide the libraries specification file from
-// dart2js.
-// TODO(sigmund): delete after all constant evaluation is done in the CFE, as
-// this data will no longer be needed on the dart2js side.
-Future<Iterable<String>> getSupportedLibraryNames(
-    Uri librariesSpecificationUri, String target,
-    {required Future<String> readJson(Uri uri)}) async {
-  return (await LibrariesSpecification.load(
-          librariesSpecificationUri, readJson))
-      .specificationFor(target)
-      .allLibraries
-      .where((l) => l.isSupported)
-      .map((l) => l.name);
-}
-
 /// Desugar API to determine whether [member] is a redirecting factory
 /// constructor.
 // TODO(sigmund): Delete this API once `member.isRedirectingFactory`
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index f2c63b83f2a..c86c3c004a7 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -69,6 +69,10 @@ abstract class LibraryBuilder implements ModifierBuilder {
   /// This is the canonical uri for the library, for instance 'dart:core'.
   Uri get importUri;
 
+  /// If true, the library is not supported through the 'dart.library.*' value
+  /// used in conditional imports and `bool.fromEnvironment` constants.
+  bool get isUnsupported;
+
   Iterator<Builder> get iterator;
 
   NameIterator get nameIterator;
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
index b3675032667..d09fac5fd91 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
@@ -130,6 +130,9 @@ class DillLibraryBuilder extends LibraryBuilderImpl {
     }
   }
 
+  @override
+  bool get isUnsupported => library.isUnsupported;
+
   @override
   bool get isSynthetic => library.isSynthetic;
 
diff --git a/pkg/front_end/lib/src/fasta/incremental_compiler.dart b/pkg/front_end/lib/src/fasta/incremental_compiler.dart
index 6e012464579..8152374d4ec 100644
--- a/pkg/front_end/lib/src/fasta/incremental_compiler.dart
+++ b/pkg/front_end/lib/src/fasta/incremental_compiler.dart
@@ -1735,6 +1735,7 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
         loader: lastGoodKernelTarget.loader,
         scope: libraryBuilder.scope.createNestedScope("expression"),
         nameOrigin: libraryBuilder,
+        isUnsupported: libraryBuilder.isUnsupported,
       );
       _ticker.logMs("Created debug library");
 
diff --git a/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart b/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
index f9bfef439b0..63f0ffdacfa 100644
--- a/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart
@@ -39,8 +39,8 @@ import 'constant_int_folder.dart';
 part 'constant_collection_builders.dart';
 
 Component transformComponent(
+    Target target,
     Component component,
-    ConstantsBackend backend,
     Map<String, String> environmentDefines,
     ErrorReporter errorReporter,
     EvaluationMode evaluationMode,
@@ -70,7 +70,7 @@ Component transformComponent(
   final TypeEnvironment typeEnvironment =
       new TypeEnvironment(coreTypes, hierarchy);
 
-  transformLibraries(component.libraries, backend, environmentDefines,
+  transformLibraries(component, component.libraries, target, environmentDefines,
       typeEnvironment, errorReporter, evaluationMode,
       enableTripleShift: enableTripleShift,
       enableConstFunctions: enableConstFunctions,
@@ -81,8 +81,9 @@ Component transformComponent(
 }
 
 ConstantEvaluationData transformLibraries(
+    Component component,
     List<Library> libraries,
-    ConstantsBackend backend,
+    Target target,
     Map<String, String>? environmentDefines,
     TypeEnvironment typeEnvironment,
     ErrorReporter errorReporter,
@@ -103,13 +104,14 @@ ConstantEvaluationData transformLibraries(
   // ignore: unnecessary_null_comparison
   assert(enableConstructorTearOff != null);
   final ConstantsTransformer constantsTransformer = new ConstantsTransformer(
-      backend,
+      target,
       environmentDefines,
       evaluateAnnotations,
       enableTripleShift,
       enableConstFunctions,
       enableConstructorTearOff,
       errorOnUnevaluatedConstant,
+      component,
       typeEnvironment,
       errorReporter,
       evaluationMode);
@@ -124,7 +126,8 @@ ConstantEvaluationData transformLibraries(
 
 void transformProcedure(
     Procedure procedure,
-    ConstantsBackend backend,
+    Target target,
+    Component component,
     Map<String, String>? environmentDefines,
     TypeEnvironment typeEnvironment,
     ErrorReporter errorReporter,
@@ -145,13 +148,14 @@ void transformProcedure(
   // ignore: unnecessary_null_comparison
   assert(enableConstructorTearOff != null);
   final ConstantsTransformer constantsTransformer = new ConstantsTransformer(
-      backend,
+      target,
       environmentDefines,
       evaluateAnnotations,
       enableTripleShift,
       enableConstFunctions,
       enableConstructorTearOff,
       errorOnUnevaluatedConstant,
+      component,
       typeEnvironment,
       errorReporter,
       evaluationMode);
@@ -341,18 +345,25 @@ class ConstantsTransformer extends RemovingTransformer {
   final bool errorOnUnevaluatedConstant;
 
   ConstantsTransformer(
-      this.backend,
+      Target target,
       Map<String, String>? environmentDefines,
       this.evaluateAnnotations,
       this.enableTripleShift,
       this.enableConstFunctions,
       this.enableConstructorTearOff,
       this.errorOnUnevaluatedConstant,
+      Component component,
       this.typeEnvironment,
       ErrorReporter errorReporter,
       EvaluationMode evaluationMode)
-      : constantEvaluator = new ConstantEvaluator(
-            backend, environmentDefines, typeEnvironment, errorReporter,
+      : this.backend = target.constantsBackend,
+        constantEvaluator = new ConstantEvaluator(
+            target.dartLibrarySupport,
+            target.constantsBackend,
+            component,
+            environmentDefines,
+            typeEnvironment,
+            errorReporter,
             enableTripleShift: enableTripleShift,
             enableConstFunctions: enableConstFunctions,
             errorOnUnevaluatedConstant: errorOnUnevaluatedConstant,
@@ -871,11 +882,13 @@ class ConstantsTransformer extends RemovingTransformer {
 }
 
 class ConstantEvaluator implements ExpressionVisitor<Constant> {
+  final DartLibrarySupport dartLibrarySupport;
   final ConstantsBackend backend;
   final NumberSemantics numberSemantics;
   late ConstantIntFolder intFolder;
-  Map<String, String>? environmentDefines;
+  Map<String, String>? _environmentDefines;
   final bool errorOnUnevaluatedConstant;
+  final Component component;
   final CoreTypes coreTypes;
   final TypeEnvironment typeEnvironment;
   StaticTypeContext? _staticTypeContext;
@@ -914,8 +927,8 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
 
   late ConstantWeakener _weakener;
 
-  ConstantEvaluator(this.backend, this.environmentDefines, this.typeEnvironment,
-      this.errorReporter,
+  ConstantEvaluator(this.dartLibrarySupport, this.backend, this.component,
+      this._environmentDefines, this.typeEnvironment, this.errorReporter,
       {this.enableTripleShift = false,
       this.enableConstFunctions = false,
       this.errorOnUnevaluatedConstant = false,
@@ -925,7 +938,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
         canonicalizationCache = <Constant, Constant>{},
         nodeCache = <Node, Constant?>{},
         env = new EvaluationEnvironment() {
-    if (environmentDefines == null && !backend.supportsUnevaluatedConstants) {
+    if (_environmentDefines == null && !backend.supportsUnevaluatedConstants) {
       throw new ArgumentError(
           "No 'environmentDefines' passed to the constant evaluator but the "
           "ConstantsBackend does not support unevaluated constants.");
@@ -947,6 +960,43 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
     _weakener = new ConstantWeakener(this);
   }
 
+  Map<String, String>? _supportedLibrariesCache;
+
+  Map<String, String> _computeSupportedLibraries() {
+    Map<String, String> map = {};
+    for (Library library in component.libraries) {
+      if (library.importUri.scheme == 'dart') {
+        map[library.importUri.path] =
+            DartLibrarySupport.getDartLibrarySupportValue(
+                library.importUri.path,
+                libraryExists: true,
+                isSynthetic: library.isSynthetic,
+                isUnsupported: library.isUnsupported,
+                dartLibrarySupport: dartLibrarySupport);
+      }
+    }
+    return map;
+  }
+
+  String? lookupEnvironment(String key) {
+    if (DartLibrarySupport.isDartLibraryQualifier(key)) {
+      String libraryName = DartLibrarySupport.getDartLibraryName(key);
+      String? value = (_supportedLibrariesCache ??=
+          _computeSupportedLibraries())[libraryName];
+      return value ?? "";
+    }
+    return _environmentDefines![key];
+  }
+
+  bool hasEnvironmentKey(String key) {
+    if (key.startsWith(DartLibrarySupport.dartLibraryPrefix)) {
+      return true;
+    }
+    return _environmentDefines!.containsKey(key);
+  }
+
+  bool get hasEnvironment => _environmentDefines != null;
+
   DartType convertType(DartType type) {
     switch (evaluationMode) {
       case EvaluationMode.strong:
@@ -1375,7 +1425,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
     Constant constant = node.constant;
     Constant result = constant;
     if (constant is UnevaluatedConstant) {
-      if (environmentDefines != null) {
+      if (hasEnvironment) {
         result = _evaluateSubexpression(constant.expression);
         if (result is AbortConstant) return result;
       } else {
@@ -2902,7 +2952,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
 
   Constant _handleFromEnvironment(
       Procedure target, StringConstant name, Map<String, Constant> named) {
-    String? value = environmentDefines![name.value];
+    String? value = lookupEnvironment(name.value);
     Constant? defaultValue = named["defaultValue"];
     if (target.enclosingClass == coreTypes.boolClass) {
       Constant boolConstant;
@@ -2961,9 +3011,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
   }
 
   Constant _handleHasEnvironment(StringConstant name) {
-    return environmentDefines!.containsKey(name.value)
-        ? trueConstant
-        : falseConstant;
+    return hasEnvironmentKey(name.value) ? trueConstant : falseConstant;
   }
 
   @override
@@ -3015,7 +3063,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
             positionals.length == 1 &&
             (target.name.text == "fromEnvironment" ||
                 target.name.text == "hasEnvironment")) {
-          if (environmentDefines != null) {
+          if (hasEnvironment) {
             // Evaluate environment constant.
             Constant name = positionals.single;
             if (name is StringConstant) {
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 00736780467..79ff78dbe18 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -1343,8 +1343,9 @@ class KernelTarget extends TargetImplementation {
 
     constants.ConstantEvaluationData constantEvaluationData =
         constants.transformLibraries(
+            component!,
             loader.libraries,
-            backendTarget.constantsBackend,
+            backendTarget,
             environmentDefines,
             environment,
             new KernelConstantErrorReporter(loader),
@@ -1400,7 +1401,8 @@ class KernelTarget extends TargetImplementation {
 
     constants.transformProcedure(
       procedure,
-      backendTarget.constantsBackend,
+      backendTarget,
+      component!,
       environmentDefines,
       environment,
       new KernelConstantErrorReporter(loader),
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 01f02a8e415..915ff8d2a31 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -139,6 +139,9 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
 
   Uri? get packageUriForTesting => _packageUri;
 
+  @override
+  final bool isUnsupported;
+
   final List<Object> accessors = <Object>[];
 
   @override
@@ -250,7 +253,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       Library library,
       LibraryBuilder? nameOrigin,
       Library? referencesFrom,
-      bool? referenceIsPartOwner)
+      bool? referenceIsPartOwner,
+      bool isUnsupported)
       : this.fromScopes(
             loader,
             fileUri,
@@ -261,7 +265,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
             origin,
             library,
             nameOrigin,
-            referencesFrom);
+            referencesFrom,
+            isUnsupported);
 
   SourceLibraryBuilder.fromScopes(
       this.loader,
@@ -273,7 +278,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       SourceLibraryBuilder? origin,
       this.library,
       this._nameOrigin,
-      this.referencesFrom)
+      this.referencesFrom,
+      this.isUnsupported)
       : _languageVersion = packageLanguageVersion,
         currentTypeParameterScopeBuilder = _libraryTypeParameterScopeBuilder,
         referencesFromIndexed =
@@ -462,7 +468,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       Library? target,
       LibraryBuilder? nameOrigin,
       Library? referencesFrom,
-      bool? referenceIsPartOwner})
+      bool? referenceIsPartOwner,
+      required bool isUnsupported})
       : this.internal(
             loader,
             fileUri,
@@ -480,7 +487,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
                   ..setLanguageVersion(packageLanguageVersion.version)),
             nameOrigin,
             referencesFrom,
-            referenceIsPartOwner);
+            referenceIsPartOwner,
+            isUnsupported);
 
   @override
   bool get isPart => partOfName != null || partOfUri != null;
@@ -703,7 +711,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       int uriOffset) {
     if (configurations != null) {
       for (Configuration config in configurations) {
-        if (lookupImportCondition(config.dottedName) == config.condition) {
+        if (loader.getLibrarySupportValue(config.dottedName) ==
+            config.condition) {
           uri = config.importUri;
           break;
         }
@@ -717,26 +726,6 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
     exports.add(new Export(this, exportedLibrary, combinators, charOffset));
   }
 
-  String lookupImportCondition(String dottedName) {
-    const String prefix = "dart.library.";
-    if (!dottedName.startsWith(prefix)) return "";
-    dottedName = dottedName.substring(prefix.length);
-    if (!loader.target.uriTranslator.isLibrarySupported(dottedName)) return "";
-
-    LibraryBuilder? imported =
-        loader.lookupLibraryBuilder(new Uri(scheme: "dart", path: dottedName));
-
-    if (imported == null) {
-      LibraryBuilder coreLibrary = loader.readAsEntryPoint(resolve(
-          this.importUri,
-          new Uri(scheme: "dart", path: "core").toString(),
-          -1));
-      imported = coreLibrary.loader
-          .lookupLibraryBuilder(new Uri(scheme: 'dart', path: dottedName));
-    }
-    return imported != null && !imported.isSynthetic ? "true" : "";
-  }
-
   void addImport(
       List<MetadataBuilder>? metadata,
       String uri,
@@ -750,7 +739,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
       int importIndex) {
     if (configurations != null) {
       for (Configuration config in configurations) {
-        if (lookupImportCondition(config.dottedName) == config.condition) {
+        if (loader.getLibrarySupportValue(config.dottedName) ==
+            config.condition) {
           uri = config.importUri;
           break;
         }
@@ -1076,6 +1066,7 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
     if (!modifyTarget) return library;
 
     library.isSynthetic = isSynthetic;
+    library.isUnsupported = isUnsupported;
     addDependencies(library, new Set<SourceLibraryBuilder>());
 
     library.name = name;
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index afd4413a440..40412d7f223 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -26,6 +26,7 @@ import 'package:kernel/class_hierarchy.dart'
     show ClassHierarchy, HandleAmbiguousSupertypes;
 import 'package:kernel/core_types.dart' show CoreTypes;
 import 'package:kernel/reference_from_index.dart' show ReferenceFromIndex;
+import 'package:kernel/target/targets.dart';
 import 'package:kernel/type_environment.dart';
 import 'package:kernel/util/graph.dart';
 import 'package:package_config/package_config.dart' as package_config;
@@ -318,7 +319,32 @@ class SourceLoader extends Loader {
         loader: this,
         origin: origin,
         referencesFrom: referencesFrom,
-        referenceIsPartOwner: referenceIsPartOwner);
+        referenceIsPartOwner: referenceIsPartOwner,
+        isUnsupported: origin?.library.isUnsupported ??
+            importUri.scheme == 'dart' &&
+                !target.uriTranslator.isLibrarySupported(importUri.path));
+  }
+
+  /// Return `"true"` if the [dottedName] is a 'dart.library.*' qualifier for a
+  /// supported dart:* library, and `""` otherwise.
+  ///
+  /// This is used to determine conditional imports and `bool.fromEnvironment`
+  /// constant values for "dart.library.[libraryName]" values.
+  String getLibrarySupportValue(String dottedName) {
+    if (!DartLibrarySupport.isDartLibraryQualifier(dottedName)) {
+      return "";
+    }
+    String libraryName = DartLibrarySupport.getDartLibraryName(dottedName);
+    Uri uri = new Uri(scheme: "dart", path: libraryName);
+    LibraryBuilder? library = lookupLibraryBuilder(uri);
+    // TODO(johnniwinther): Why is the dill target sometimes not loaded at this
+    // point? And does it matter?
+    library ??= target.dillTarget.loader.lookupLibraryBuilder(uri);
+    return DartLibrarySupport.getDartLibrarySupportValue(libraryName,
+        libraryExists: library != null,
+        isSynthetic: library?.isSynthetic ?? true,
+        isUnsupported: library?.isUnsupported ?? true,
+        dartLibrarySupport: target.backendTarget.dartLibrarySupport);
   }
 
   SourceLibraryBuilder _createSourceLibraryBuilder(
diff --git a/pkg/front_end/lib/src/fasta/uri_translator.dart b/pkg/front_end/lib/src/fasta/uri_translator.dart
index cf624b4f7fc..dac88453a75 100644
--- a/pkg/front_end/lib/src/fasta/uri_translator.dart
+++ b/pkg/front_end/lib/src/fasta/uri_translator.dart
@@ -50,9 +50,7 @@ class UriTranslator {
   }
 
   bool isLibrarySupported(String libraryName) {
-    // TODO(sigmund): change this to `?? false` when all backends provide the
-    // `libraries.json` file by default (Issue #32657).
-    return dartLibraries.libraryInfoFor(libraryName)?.isSupported ?? true;
+    return dartLibraries.libraryInfoFor(libraryName)?.isSupported ?? false;
   }
 
   Uri? _translateDartUri(Uri uri) {
diff --git a/pkg/front_end/test/constant_evaluator_benchmark.dart b/pkg/front_end/test/constant_evaluator_benchmark.dart
index 35f5faa95eb..c86c066239b 100644
--- a/pkg/front_end/test/constant_evaluator_benchmark.dart
+++ b/pkg/front_end/test/constant_evaluator_benchmark.dart
@@ -7,43 +7,33 @@ import 'dart:typed_data' show Uint8List;
 
 import 'package:_fe_analyzer_shared/src/messages/codes.dart';
 import 'package:compiler/src/kernel/dart2js_target.dart' show Dart2jsTarget;
-
 import 'package:dev_compiler/src/kernel/target.dart' show DevCompilerTarget;
-
 import 'package:front_end/src/api_prototype/compiler_options.dart'
     show CompilerOptions, DiagnosticMessage;
-
 import 'package:front_end/src/api_prototype/experimental_flags.dart'
     show ExperimentalFlag;
-
 import 'package:front_end/src/base/processed_options.dart'
     show ProcessedOptions;
-
 import 'package:front_end/src/fasta/compiler_context.dart' show CompilerContext;
-
 import 'package:front_end/src/fasta/incremental_compiler.dart'
     show IncrementalCompiler;
 import 'package:front_end/src/fasta/kernel/constant_evaluator.dart' as constants
     show EvaluationMode, transformLibraries, ErrorReporter;
-
 import 'package:front_end/src/fasta/kernel/kernel_target.dart';
+import 'package:front_end/src/fasta/kernel/utils.dart' show serializeComponent;
 import 'package:kernel/ast.dart';
 import 'package:kernel/binary/ast_from_binary.dart';
-import 'package:kernel/core_types.dart';
 import 'package:kernel/class_hierarchy.dart';
+import 'package:kernel/core_types.dart';
 import 'package:kernel/target/changed_structure_notifier.dart';
 import 'package:kernel/target/targets.dart'
-    show ConstantsBackend, DiagnosticReporter, Target, TargetFlags;
+    show DiagnosticReporter, Target, TargetFlags;
 import 'package:kernel/type_environment.dart';
-
 import "package:vm/target/flutter.dart" show FlutterTarget;
-
 import "package:vm/target/vm.dart" show VmTarget;
 
 import 'incremental_suite.dart' show getOptions;
 
-import 'package:front_end/src/fasta/kernel/utils.dart' show serializeComponent;
-
 bool? tryWithNoEnvironment;
 bool verbose = false;
 bool skipNonNullEnvironment = false;
@@ -80,8 +70,6 @@ void benchmark(Component component, List<Library> libraries) {
 
         stopwatch.reset();
         CoreTypes coreTypes = new CoreTypes(component);
-        ConstantsBackend constantsBackend =
-            target.backendTarget.constantsBackend;
         ClassHierarchy hierarchy = new ClassHierarchy(component, coreTypes);
         TypeEnvironment environment = new TypeEnvironment(coreTypes, hierarchy);
         if (verbose) {
@@ -91,8 +79,9 @@ void benchmark(Component component, List<Library> libraries) {
 
         stopwatch.reset();
         constants.transformLibraries(
+            component,
             component.libraries,
-            constantsBackend,
+            target.backendTarget,
             environmentDefines,
             environment,
             new SilentErrorReporter(),
diff --git a/pkg/front_end/test/fasta/generator_to_string_test.dart b/pkg/front_end/test/fasta/generator_to_string_test.dart
index f4790b9ad08..787ad86e8c0 100644
--- a/pkg/front_end/test/fasta/generator_to_string_test.dart
+++ b/pkg/front_end/test/fasta/generator_to_string_test.dart
@@ -94,7 +94,8 @@ Future<void> main() async {
                 new DillTarget(c.options.ticker, uriTranslator,
                     new NoneTarget(new TargetFlags())),
                 uriTranslator)
-            .loader);
+            .loader,
+        isUnsupported: false);
     libraryBuilder.markLanguageVersionFinal();
     LoadLibraryBuilder loadLibraryBuilder =
         new LoadLibraryBuilder(libraryBuilder, dummyLibraryDependency, -1);
diff --git a/pkg/front_end/test/fasta/testing/suite.dart b/pkg/front_end/test/fasta/testing/suite.dart
index f7221fd8689..2c4da283955 100644
--- a/pkg/front_end/test/fasta/testing/suite.dart
+++ b/pkg/front_end/test/fasta/testing/suite.dart
@@ -5,91 +5,63 @@
 library fasta.testing.suite;
 
 import 'dart:convert' show jsonDecode, utf8;
-
 import 'dart:io' show Directory, File, Platform;
-
 import 'dart:typed_data' show Uint8List;
 
 import 'package:_fe_analyzer_shared/src/scanner/token.dart'
     show LanguageVersionToken, Token;
-
 import 'package:_fe_analyzer_shared/src/util/colors.dart' as colors;
 import 'package:_fe_analyzer_shared/src/util/libraries_specification.dart'
     show LibraryInfo;
 import 'package:_fe_analyzer_shared/src/util/options.dart';
 import 'package:compiler/src/kernel/dart2js_target.dart';
 import 'package:dev_compiler/src/kernel/target.dart';
-
 import 'package:front_end/src/api_prototype/compiler_options.dart'
     show
         CompilerOptions,
         DiagnosticMessage,
         parseExperimentalArguments,
         parseExperimentalFlags;
-
 import 'package:front_end/src/api_prototype/constant_evaluator.dart'
     show ConstantEvaluator, ErrorReporter, EvaluationMode;
-
 import 'package:front_end/src/api_prototype/experimental_flags.dart'
     show
         AllowedExperimentalFlags,
         ExperimentalFlag,
         defaultAllowedExperimentalFlags,
         isExperimentEnabled;
-
 import 'package:front_end/src/api_prototype/file_system.dart'
     show FileSystem, FileSystemEntity, FileSystemException;
-
 import 'package:front_end/src/api_prototype/incremental_kernel_generator.dart'
     show IncrementalCompilerResult;
-
 import 'package:front_end/src/api_prototype/standard_file_system.dart'
     show StandardFileSystem;
-
+import 'package:front_end/src/base/command_line_options.dart';
+import 'package:front_end/src/base/nnbd_mode.dart' show NnbdMode;
 import 'package:front_end/src/base/processed_options.dart'
     show ProcessedOptions;
-
 import 'package:front_end/src/compute_platform_binaries_location.dart'
     show computePlatformBinariesLocation, computePlatformDillName;
-
-import 'package:front_end/src/base/command_line_options.dart';
-
-import 'package:front_end/src/base/nnbd_mode.dart' show NnbdMode;
-
 import 'package:front_end/src/fasta/builder/library_builder.dart'
     show LibraryBuilder;
-
 import 'package:front_end/src/fasta/compiler_context.dart' show CompilerContext;
-
 import 'package:front_end/src/fasta/dill/dill_target.dart' show DillTarget;
-
 import 'package:front_end/src/fasta/incremental_compiler.dart'
     show IncrementalCompiler;
-
 import 'package:front_end/src/fasta/kernel/hierarchy/hierarchy_builder.dart'
     show ClassHierarchyBuilder;
-
 import 'package:front_end/src/fasta/kernel/hierarchy/hierarchy_node.dart'
     show ClassHierarchyNode;
-
 import 'package:front_end/src/fasta/kernel/kernel_target.dart'
     show KernelTarget;
-
 import 'package:front_end/src/fasta/kernel/utils.dart' show ByteSink;
-
-import 'package:front_end/src/fasta/messages.dart' show LocatedMessage;
-
-import 'package:front_end/src/fasta/ticker.dart' show Ticker;
-
-import 'package:front_end/src/fasta/uri_translator.dart' show UriTranslator;
-
 import 'package:front_end/src/fasta/kernel/verifier.dart' show verifyComponent;
-
+import 'package:front_end/src/fasta/messages.dart' show LocatedMessage;
+import 'package:front_end/src/fasta/ticker.dart' show Ticker;
+import 'package:front_end/src/fasta/uri_translator.dart' show UriTranslator;
 import 'package:front_end/src/fasta/util/parser_ast.dart'
     show ParserAstVisitor, getAST;
-
 import 'package:front_end/src/fasta/util/parser_ast_helper.dart';
-
 import 'package:kernel/ast.dart'
     show
         AwaitExpression,
@@ -111,24 +83,16 @@ import 'package:kernel/ast.dart'
         Version,
         Visitor,
         VisitorVoidMixin;
-
 import 'package:kernel/binary/ast_to_binary.dart' show BinaryPrinter;
-
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
-
 import 'package:kernel/core_types.dart' show CoreTypes;
-
 import 'package:kernel/kernel.dart'
     show RecursiveResultVisitor, loadComponentFromBytes;
-
 import 'package:kernel/reference_from_index.dart' show ReferenceFromIndex;
-
 import 'package:kernel/target/changed_structure_notifier.dart'
     show ChangedStructureNotifier;
-
 import 'package:kernel/target/targets.dart'
     show
-        ConstantsBackend,
         DiagnosticReporter,
         NoneConstantsBackend,
         NoneTarget,
@@ -137,10 +101,8 @@ import 'package:kernel/target/targets.dart'
         TestTargetFlags,
         TestTargetMixin,
         TestTargetWrapper;
-
 import 'package:kernel/type_environment.dart'
     show StaticTypeContext, TypeEnvironment;
-
 import 'package:testing/testing.dart'
     show
         Chain,
@@ -151,11 +113,9 @@ import 'package:testing/testing.dart'
         Step,
         TestDescription,
         StdioProcess;
-
 import 'package:vm/target/vm.dart' show VmTarget;
 
 import '../../testing_utils.dart' show checkEnvironment;
-
 import '../../utils/kernel_chain.dart'
     show
         ComponentResult,
@@ -165,7 +125,6 @@ import '../../utils/kernel_chain.dart'
         Print,
         TypeCheck,
         WriteDill;
-
 import '../../utils/validating_instrumentation.dart'
     show ValidatingInstrumentation;
 
@@ -901,12 +860,12 @@ class StressConstantEvaluatorStep
   Future<Result<ComponentResult>> run(
       ComponentResult result, FastaContext context) {
     KernelTarget target = result.sourceTarget;
-    ConstantsBackend constantsBackend = target.backendTarget.constantsBackend;
     TypeEnvironment environment =
         new TypeEnvironment(target.loader.coreTypes, target.loader.hierarchy);
     StressConstantEvaluatorVisitor stressConstantEvaluatorVisitor =
         new StressConstantEvaluatorVisitor(
-      constantsBackend,
+      target.backendTarget,
+      result.component,
       result.options.environmentDefines,
       target.isExperimentEnabledGlobally(ExperimentalFlag.tripleShift),
       environment,
@@ -937,7 +896,8 @@ class StressConstantEvaluatorVisitor extends RecursiveResultVisitor<Node>
   List<String> output = [];
 
   StressConstantEvaluatorVisitor(
-      ConstantsBackend backend,
+      Target target,
+      Component component,
       Map<String, String>? environmentDefines,
       bool enableTripleShift,
       TypeEnvironment typeEnvironment,
@@ -945,12 +905,22 @@ class StressConstantEvaluatorVisitor extends RecursiveResultVisitor<Node>
       bool errorOnUnevaluatedConstant,
       EvaluationMode evaluationMode) {
     constantEvaluator = new ConstantEvaluator(
-        backend, environmentDefines, typeEnvironment, this,
+        target.dartLibrarySupport,
+        target.constantsBackend,
+        component,
+        environmentDefines,
+        typeEnvironment,
+        this,
         enableTripleShift: enableTripleShift,
         errorOnUnevaluatedConstant: errorOnUnevaluatedConstant,
         evaluationMode: evaluationMode);
     constantEvaluatorWithEmptyEnvironment = new ConstantEvaluator(
-        backend, {}, typeEnvironment, this,
+        target.dartLibrarySupport,
+        target.constantsBackend,
+        component,
+        {},
+        typeEnvironment,
+        this,
         enableTripleShift: enableTripleShift,
         errorOnUnevaluatedConstant: errorOnUnevaluatedConstant,
         evaluationMode: evaluationMode);
@@ -990,7 +960,7 @@ class StressConstantEvaluatorVisitor extends RecursiveResultVisitor<Node>
         }
       }
       if (!evaluate) return node;
-      if (constantEvaluator.environmentDefines != null) {
+      if (constantEvaluator.hasEnvironment) {
         throw "Unexpected UnevaluatedConstant "
             "when the environment is not null.";
       }
@@ -1010,7 +980,7 @@ class StressConstantEvaluatorVisitor extends RecursiveResultVisitor<Node>
     bool evaluatedWithEmptyEnvironment = false;
     if (x is UnevaluatedConstant && x.expression is! InvalidExpression) {
       // try with an environment
-      if (constantEvaluator.environmentDefines != null) {
+      if (constantEvaluator.hasEnvironment) {
         throw "Unexpected UnevaluatedConstant (with an InvalidExpression in "
             "it) when the environment is not null.";
       }
@@ -1760,6 +1730,8 @@ Target createTarget(FolderOptions folderOptions, FastaContext context) {
     forceConstructorTearOffLoweringForTesting:
         folderOptions.forceConstructorTearOffLowering,
     enableNullSafety: context.soundNullSafety,
+    supportedDartLibraries: {'_supported.by.target'},
+    unsupportedDartLibraries: {'unsupported.by.target'},
   );
   Target target;
   switch (folderOptions.target) {
diff --git a/pkg/front_end/test/spell_checking_list_code.txt b/pkg/front_end/test/spell_checking_list_code.txt
index 8b9b65b4047..7cfd23ad122 100644
--- a/pkg/front_end/test/spell_checking_list_code.txt
+++ b/pkg/front_end/test/spell_checking_list_code.txt
@@ -688,6 +688,7 @@ jakemac
 java
 jenkins
 jensj
+jit
 job
 johnniwinther
 js
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart b/pkg/front_end/testcases/dart2js/conditional_import.dart
index 61224510565..58ccbe77fc8 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart
@@ -8,7 +8,11 @@ import "conditional_import.dart"
     if (dart.library.html) "dart:html" as a;
 
 // All three libraries have an HttpRequest class.
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
@@ -20,6 +24,13 @@ testA(a.HttpRequest request) {
 }
 
 testB(b.HttpRequest request) {
+  request.certificate; // error (from dart:io)
+  request.response; // ok (from dart:io and dart:html)
+  request.readyState; // ok (from dart:html)
+  request.hashCode; // ok
+}
+
+testC(c.HttpRequest request) {
   request.certificate; // error
   request.response; // error
   request.readyState; // error
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.expect
index 5f33ec12d1d..560176d068f 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,12 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
+import "dart:_interceptors" as _in;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +48,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
-  request.{core::Object::hashCode}{core::int};
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +88,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +98,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.transformed.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.transformed.expect
index 5f33ec12d1d..560176d068f 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.strong.transformed.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,12 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
+import "dart:_interceptors" as _in;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +48,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
-  request.{core::Object::hashCode}{core::int};
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +88,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +98,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline.expect
index dc1072741fe..a62da80b3d2 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline.expect
@@ -1,11 +1,15 @@
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
 expect(expected, actual) {}
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline_modelled.expect
index 6e90f3d3e27..7af9c6fc6c5 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.textual_outline_modelled.expect
@@ -1,4 +1,7 @@
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
@@ -8,4 +11,5 @@ class HttpRequest {}
 expect(expected, actual) {}
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.expect
index 5f33ec12d1d..560176d068f 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,12 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
+import "dart:_interceptors" as _in;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +48,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
-  request.{core::Object::hashCode}{core::int};
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +88,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +98,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.modular.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.modular.expect
index 5f33ec12d1d..560176d068f 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.modular.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,12 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
+import "dart:_interceptors" as _in;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +48,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
-  request.{core::Object::hashCode}{core::int};
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +88,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +98,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.outline.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.outline.expect
index 8e7ea0fbe1a..b5be69ac5b9 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.outline.expect
@@ -1,10 +1,11 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -12,9 +13,11 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic
+static method testA(html::HttpRequest request) → dynamic
   ;
-static method testB(self::HttpRequest request) → dynamic
+static method testB(html::HttpRequest request) → dynamic
+  ;
+static method testC(self::HttpRequest request) → dynamic
   ;
 static method main() → void
   ;
diff --git a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.transformed.expect b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.transformed.expect
index 5f33ec12d1d..560176d068f 100644
--- a/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/dart2js/conditional_import.dart.weak.transformed.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,12 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
+import "dart:_interceptors" as _in;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +48,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
-  request.{core::Object::hashCode}{core::int};
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{_in::Interceptor::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dart2js/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dart2js/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +88,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +98,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart b/pkg/front_end/testcases/dartdevc/conditional_import.dart
index 61224510565..58ccbe77fc8 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart
@@ -8,7 +8,11 @@ import "conditional_import.dart"
     if (dart.library.html) "dart:html" as a;
 
 // All three libraries have an HttpRequest class.
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
@@ -20,6 +24,13 @@ testA(a.HttpRequest request) {
 }
 
 testB(b.HttpRequest request) {
+  request.certificate; // error (from dart:io)
+  request.response; // ok (from dart:io and dart:html)
+  request.readyState; // ok (from dart:html)
+  request.hashCode; // ok
+}
+
+testC(c.HttpRequest request) {
   request.certificate; // error
   request.response; // error
   request.readyState; // error
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.expect
index 3b668f735de..efcac9a64fe 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,11 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +47,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +87,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +97,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.transformed.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.transformed.expect
index 3b668f735de..efcac9a64fe 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.strong.transformed.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,11 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +47,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +87,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +97,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline.expect
index dc1072741fe..a62da80b3d2 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline.expect
@@ -1,11 +1,15 @@
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
 expect(expected, actual) {}
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline_modelled.expect
index 6e90f3d3e27..7af9c6fc6c5 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.textual_outline_modelled.expect
@@ -1,4 +1,7 @@
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
@@ -8,4 +11,5 @@ class HttpRequest {}
 expect(expected, actual) {}
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.expect
index 3b668f735de..efcac9a64fe 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,11 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +47,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +87,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +97,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.modular.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.modular.expect
index 3b668f735de..efcac9a64fe 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.modular.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,11 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +47,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +87,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +97,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.outline.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.outline.expect
index 8e7ea0fbe1a..b5be69ac5b9 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.outline.expect
@@ -1,10 +1,11 @@
 library /*isNonNullableByDefault*/;
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -12,9 +13,11 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic
+static method testA(html::HttpRequest request) → dynamic
   ;
-static method testB(self::HttpRequest request) → dynamic
+static method testB(html::HttpRequest request) → dynamic
+  ;
+static method testC(self::HttpRequest request) → dynamic
   ;
 static method main() → void
   ;
diff --git a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.transformed.expect b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.transformed.expect
index 3b668f735de..efcac9a64fe 100644
--- a/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/dartdevc/conditional_import.dart.weak.transformed.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
-//  - 'HttpRequest' is from 'dart:_http'.
-// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-//   request.readyState; // ok (from dart:html)
-//           ^^^^^^^^^^
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:html'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+//   request.certificate; // error (from dart:io)
+//           ^^^^^^^^^^^
+//
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -28,11 +34,11 @@ library /*isNonNullableByDefault*/;
 //
 import self as self;
 import "dart:core" as core;
-import "dart:_http" as _ht;
-import "dart:io" as io;
+import "dart:html" as html;
 
-import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:html" as a;
+import "dart:html" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -41,28 +47,38 @@ class HttpRequest extends core::Object {
   static method _#new#tearOff() → self::HttpRequest
     return new self::HttpRequest::•();
 }
-static method testA(_ht::HttpRequest request) → dynamic {
-  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
-  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
- - 'HttpRequest' is from 'dart:_http'.
-Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
-  request.readyState; // ok (from dart:html)
-          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+static method testA(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:20:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(html::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:27:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:html'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
+  request.certificate; // error (from dart:io)
+          ^^^^^^^^^^^" in request{<unresolved>}.certificate;
+  request.{html::HttpRequest::response}{dynamic};
+  request.{html::HttpRequest::readyState}{core::int};
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/dartdevc/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/dartdevc/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -71,7 +87,7 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(false, #C1);
-  self::expect(true, #C1);
+  self::expect(true, #C2);
   self::expect(false, #C1);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
@@ -81,4 +97,5 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 
 constants  {
   #C1 = false
+  #C2 = true
 }
diff --git a/pkg/front_end/testcases/general/conditional_import.dart b/pkg/front_end/testcases/general/conditional_import.dart
index 067c7845a0f..648ea9b8e99 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart
+++ b/pkg/front_end/testcases/general/conditional_import.dart
@@ -8,7 +8,11 @@ import "conditional_import.dart"
     if (dart.library.html) "dart:html" as a;
 
 // All three libraries have an HttpRequest class.
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
@@ -20,6 +24,13 @@ testA(a.HttpRequest request) {
 }
 
 testB(b.HttpRequest request) {
+  request.certificate; // ok (from dart:io)
+  request.response; // ok (from dart:io and dart:html)
+  request.readyState; // error (from dart:html)
+  request.hashCode; // ok
+}
+
+testC(c.HttpRequest request) {
   request.certificate; // error
   request.response; // error
   request.readyState; // error
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.textual_outline.expect b/pkg/front_end/testcases/general/conditional_import.dart.textual_outline.expect
index dc1072741fe..a62da80b3d2 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.textual_outline.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.textual_outline.expect
@@ -1,11 +1,15 @@
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
 
 class HttpRequest {}
 
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
 expect(expected, actual) {}
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/general/conditional_import.dart.textual_outline_modelled.expect
index 6e90f3d3e27..7af9c6fc6c5 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.textual_outline_modelled.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.textual_outline_modelled.expect
@@ -1,4 +1,7 @@
-import "conditional_import.dart" if (dart.library.foo) "dart:foo" as b;
+import "conditional_import.dart" if (dart.library.foo) "dart:foo" as c;
+import "conditional_import.dart"
+    if (dart.library.html) "dart:html"
+    if (dart.library.io) "dart:io" as b;
 import "conditional_import.dart"
     if (dart.library.io) "dart:io"
     if (dart.library.html) "dart:html" as a;
@@ -8,4 +11,5 @@ class HttpRequest {}
 expect(expected, actual) {}
 testA(a.HttpRequest request) {}
 testB(b.HttpRequest request) {}
+testC(c.HttpRequest request) {}
 void main() {}
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.weak.expect b/pkg/front_end/testcases/general/conditional_import.dart.weak.expect
index 0ae3432d4cd..00999c9078b 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.weak.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.weak.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'dart:_http'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error (from dart:html)
 //           ^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:_http'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+//   request.readyState; // error (from dart:html)
+//           ^^^^^^^^^^
+//
+// pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -32,7 +38,8 @@ import "dart:_http" as _ht;
 import "dart:io" as io;
 
 import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:io" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -42,25 +49,35 @@ class HttpRequest extends core::Object {
 static method testA(_ht::HttpRequest request) → dynamic {
   request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
   request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'dart:_http'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error (from dart:html)
           ^^^^^^^^^^" in request{<unresolved>}.readyState;
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(_ht::HttpRequest request) → dynamic {
+  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
+  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:_http'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+  request.readyState; // error (from dart:html)
+          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -69,8 +86,8 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(true, #C1);
-  self::expect(false, #C1);
-  self::expect(false, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
   if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
@@ -78,5 +95,6 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
+  #C2 = false
 }
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.weak.modular.expect b/pkg/front_end/testcases/general/conditional_import.dart.weak.modular.expect
index 0ae3432d4cd..00999c9078b 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.weak.modular.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'dart:_http'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error (from dart:html)
 //           ^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:_http'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+//   request.readyState; // error (from dart:html)
+//           ^^^^^^^^^^
+//
+// pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -32,7 +38,8 @@ import "dart:_http" as _ht;
 import "dart:io" as io;
 
 import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:io" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -42,25 +49,35 @@ class HttpRequest extends core::Object {
 static method testA(_ht::HttpRequest request) → dynamic {
   request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
   request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'dart:_http'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error (from dart:html)
           ^^^^^^^^^^" in request{<unresolved>}.readyState;
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(_ht::HttpRequest request) → dynamic {
+  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
+  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:_http'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+  request.readyState; // error (from dart:html)
+          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -69,8 +86,8 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(true, #C1);
-  self::expect(false, #C1);
-  self::expect(false, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
   if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
@@ -78,5 +95,6 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
+  #C2 = false
 }
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.weak.outline.expect b/pkg/front_end/testcases/general/conditional_import.dart.weak.outline.expect
index 17e396c0d75..98d76441bdb 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.weak.outline.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.weak.outline.expect
@@ -4,7 +4,8 @@ import "dart:core" as core;
 import "dart:_http" as _ht;
 
 import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:io" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -12,7 +13,9 @@ class HttpRequest extends core::Object {
 }
 static method testA(_ht::HttpRequest request) → dynamic
   ;
-static method testB(self::HttpRequest request) → dynamic
+static method testB(_ht::HttpRequest request) → dynamic
+  ;
+static method testC(self::HttpRequest request) → dynamic
   ;
 static method main() → void
   ;
diff --git a/pkg/front_end/testcases/general/conditional_import.dart.weak.transformed.expect b/pkg/front_end/testcases/general/conditional_import.dart.weak.transformed.expect
index 0ae3432d4cd..00999c9078b 100644
--- a/pkg/front_end/testcases/general/conditional_import.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/general/conditional_import.dart.weak.transformed.expect
@@ -2,25 +2,31 @@ library /*isNonNullableByDefault*/;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'dart:_http'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error (from dart:html)
 //           ^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+//  - 'HttpRequest' is from 'dart:_http'.
+// Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+//   request.readyState; // error (from dart:html)
+//           ^^^^^^^^^^
+//
+// pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
 //   request.certificate; // error
 //           ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
 //   request.response; // error
 //           ^^^^^^^^
 //
-// pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+// pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
 //  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 // Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
 //   request.readyState; // error
@@ -32,7 +38,8 @@ import "dart:_http" as _ht;
 import "dart:io" as io;
 
 import "dart:io" as a;
-import "org-dartlang-testcase:///conditional_import.dart" as b;
+import "dart:io" as b;
+import "org-dartlang-testcase:///conditional_import.dart" as c;
 
 class HttpRequest extends core::Object {
   synthetic constructor •() → self::HttpRequest
@@ -42,25 +49,35 @@ class HttpRequest extends core::Object {
 static method testA(_ht::HttpRequest request) → dynamic {
   request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
   request.{_ht::HttpRequest::response}{_ht::HttpResponse};
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:18:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:22:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'dart:_http'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error (from dart:html)
           ^^^^^^^^^^" in request{<unresolved>}.readyState;
   request.{core::Object::hashCode}{core::int};
 }
-static method testB(self::HttpRequest request) → dynamic {
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:23:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
+static method testB(_ht::HttpRequest request) → dynamic {
+  request.{_ht::HttpRequest::certificate}{io::X509Certificate?};
+  request.{_ht::HttpRequest::response}{_ht::HttpResponse};
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:29:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+ - 'HttpRequest' is from 'dart:_http'.
+Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
+  request.readyState; // error (from dart:html)
+          ^^^^^^^^^^" in request{<unresolved>}.readyState;
+  request.{core::Object::hashCode}{core::int};
+}
+static method testC(self::HttpRequest request) → dynamic {
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:34:11: Error: The getter 'certificate' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'certificate'.
   request.certificate; // error
           ^^^^^^^^^^^" in request{<unresolved>}.certificate;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:24:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:35:11: Error: The getter 'response' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'response'.
   request.response; // error
           ^^^^^^^^" in request{<unresolved>}.response;
-  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:25:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
+  invalid-expression "pkg/front_end/testcases/general/conditional_import.dart:36:11: Error: The getter 'readyState' isn't defined for the class 'HttpRequest'.
  - 'HttpRequest' is from 'pkg/front_end/testcases/general/conditional_import.dart'.
 Try correcting the name to the name of an existing getter, or defining a getter or field named 'readyState'.
   request.readyState; // error
@@ -69,8 +86,8 @@ Try correcting the name to the name of an existing getter, or defining a getter
 }
 static method main() → void {
   self::expect(true, #C1);
-  self::expect(false, #C1);
-  self::expect(false, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
 }
 static method expect(dynamic expected, dynamic actual) → dynamic {
   if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
@@ -78,5 +95,6 @@ static method expect(dynamic expected, dynamic actual) → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
+  #C2 = false
 }
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_default_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_default_lib.dart
new file mode 100644
index 00000000000..00ee7692644
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_default_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'default';
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_supported.by.spec_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_supported.by.spec_lib.dart
new file mode 100644
index 00000000000..1c3a6c71716
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_supported.by.spec_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'supported.by.spec';
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_supported.by.target_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_supported.by.target_lib.dart
new file mode 100644
index 00000000000..36d5727c331
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_supported.by.target_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'supported.by.target';
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_internal_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_internal_lib.dart
new file mode 100644
index 00000000000..27ca30346e3
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_internal_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'unsupported.by.spec_internal';
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_lib.dart
new file mode 100644
index 00000000000..3b9f7f276bf
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.spec_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'unsupported.by.spec';
diff --git a/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.target_lib.dart b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.target_lib.dart
new file mode 100644
index 00000000000..d0216e75d11
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/import_unsupported.by.target_lib.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+String field = 'unsupported.by.target';
diff --git a/pkg/front_end/testcases/general/supported_libraries/libraries.json b/pkg/front_end/testcases/general/supported_libraries/libraries.json
new file mode 100644
index 00000000000..13d244b84ac
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/libraries.json
@@ -0,0 +1,23 @@
+{
+  "none": {
+    "libraries": {
+      "supported.by.spec": {
+        "uri": "supported.by.spec_lib.dart"
+      },
+      "_supported.by.target": {
+        "uri": "supported.by.target_lib.dart"
+      },
+      "unsupported.by.spec": {
+        "uri": "unsupported.by.spec_lib.dart",
+        "supported": false
+      },
+      "unsupported.by.target": {
+        "uri": "unsupported.by.target_lib.dart",
+        "supported": true
+      },
+      "_unsupported.by.spec_internal": {
+        "uri": "unsupported.by.spec_internal_lib.dart"
+      }
+    }
+  }
+}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart b/pkg/front_end/testcases/general/supported_libraries/main.dart
new file mode 100644
index 00000000000..0677973621b
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart
@@ -0,0 +1,63 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:supported.by.spec';
+import 'dart:unsupported.by.spec';
+import 'dart:unsupported.by.target';
+
+import 'import_default_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    as from_supported_by_spec_first;
+
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    as from_supported_by_target;
+
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    as from_supported_by_spec_last;
+
+main() {
+  supportedBySpec();
+  supportedByTarget(); // Exported through dart:supported.by.spec
+  unsupportedBySpec();
+  unsupportedByTarget();
+  unsupportedBySpecInternal(); // Exported through dart:unsupported.by.spec
+
+  expect('supported.by.spec', from_supported_by_spec_first.field);
+  expect('supported.by.target', from_supported_by_target.field);
+  expect('supported.by.spec', from_supported_by_spec_last.field);
+
+  // `dart:supported.by.spec` is supported by the libraries specification.
+  expect(true, const bool.fromEnvironment('dart.library.supported.by.spec'));
+  // `dart:_supported.by.target` is internal and therefore not supported by
+  // the libraries specification, but the test target supports it explicitly.
+  expect(true, const bool.fromEnvironment('dart.library._supported.by.target'));
+  // `dart:unsupported.by.spec` is unsupported by the libraries specification.
+  expect(false, const bool.fromEnvironment('dart.library.unsupported.by.spec'));
+  // `dart:unsupported.by.target` is unsupported by the libraries specification,
+  // but the test target explicitly marks it as unsupported.
+  expect(
+      false, const bool.fromEnvironment('dart.library.unsupported.by.target'));
+  // `dart:_unsupported.by.spec_internal` is internal and therefore not
+  // supported by the libraries specification.
+  expect(false,
+      const bool.fromEnvironment('dart.library._unsupported.by.spec_internal'));
+}
+
+expect(expected, actual) {
+  if (expected != actual) throw 'Expected $expected, actual $actual';
+}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline.expect
new file mode 100644
index 00000000000..da8d39f08a0
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline.expect
@@ -0,0 +1,27 @@
+import 'dart:supported.by.spec';
+import 'dart:unsupported.by.spec';
+import 'dart:unsupported.by.target';
+import 'import_default_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    as from_supported_by_spec_first;
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    as from_supported_by_target;
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    as from_supported_by_spec_last;
+
+main() {}
+expect(expected, actual) {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..29001ab6cf0
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.textual_outline_modelled.expect
@@ -0,0 +1,27 @@
+import 'dart:supported.by.spec';
+import 'dart:unsupported.by.spec';
+import 'dart:unsupported.by.target';
+import 'import_default_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    as from_supported_by_spec_first;
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    as from_supported_by_target;
+import 'import_default_lib.dart'
+    if (dart.library.unsupported.by.spec) 'import_unsupported.by.spec_lib.dart'
+    if (dart.library.unsupported.by.target) 'import_unsupported.by.target_lib.dart'
+    if (dart.library._unsupported.by.spec_internal) 'import_unsupported.by.spec_internal_lib.dart'
+    if (dart.library.supported.by.spec) 'import_supported.by.spec_lib.dart'
+    if (dart.library._supported.by.target) 'import_supported.by.target_lib.dart'
+    as from_supported_by_spec_last;
+
+expect(expected, actual) {}
+main() {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.expect
new file mode 100644
index 00000000000..85a3cb75f71
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.expect
@@ -0,0 +1,87 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:supported.by.spec" as spec;
+import "dart:_supported.by.target" as by_;
+import "dart:unsupported.by.spec" as spec2;
+import "dart:unsupported.by.target" as tar;
+import "dart:_unsupported.by.spec_internal" as spe;
+import "import_supported.by.spec_lib.dart" as spe2;
+import "import_supported.by.target_lib.dart" as tar2;
+import "dart:core" as core;
+
+import "dart:supported.by.spec";
+import "dart:unsupported.by.spec";
+import "dart:unsupported.by.target";
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_first;
+import "org-dartlang-testcase:///import_supported.by.target_lib.dart" as from_supported_by_target;
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_last;
+
+static method main() → dynamic {
+  spec::supportedBySpec();
+  by_::supportedByTarget();
+  spec2::unsupportedBySpec();
+  tar::unsupportedByTarget();
+  spe::unsupportedBySpecInternal();
+  self::expect("supported.by.spec", spe2::field);
+  self::expect("supported.by.target", tar2::field);
+  self::expect("supported.by.spec", spe2::field);
+  self::expect(true, #C1);
+  self::expect(true, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
+
+library dart.supported.by.spec /*isNonNullableByDefault*/;
+import self as spec;
+import "dart:_supported.by.target" as by_;
+additionalExports = (by_::supportedByTarget)
+
+export "dart:_supported.by.target";
+
+static method supportedBySpec() → void {}
+
+library dart.unsupported.by.spec /*isUnsupported,isNonNullableByDefault*/;
+import self as spec2;
+import "dart:_unsupported.by.spec_internal" as spe;
+additionalExports = (spe::unsupportedBySpecInternal)
+
+export "dart:_unsupported.by.spec_internal";
+
+static method unsupportedBySpec() → void {}
+
+library dart.unsupported.by.target /*isNonNullableByDefault*/;
+import self as tar;
+
+static method unsupportedByTarget() → void {}
+
+library /*isNonNullableByDefault*/;
+import self as spe2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.spec";
+
+library /*isNonNullableByDefault*/;
+import self as tar2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.target";
+
+library dart._supported.by_target /*isUnsupported,isNonNullableByDefault*/;
+import self as by_;
+
+static method supportedByTarget() → void {}
+
+library dart._unsupported.by.spec_internal /*isUnsupported,isNonNullableByDefault*/;
+import self as spe;
+
+static method unsupportedBySpecInternal() → void {}
+
+constants  {
+  #C1 = true
+  #C2 = false
+}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.modular.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.modular.expect
new file mode 100644
index 00000000000..85a3cb75f71
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.modular.expect
@@ -0,0 +1,87 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:supported.by.spec" as spec;
+import "dart:_supported.by.target" as by_;
+import "dart:unsupported.by.spec" as spec2;
+import "dart:unsupported.by.target" as tar;
+import "dart:_unsupported.by.spec_internal" as spe;
+import "import_supported.by.spec_lib.dart" as spe2;
+import "import_supported.by.target_lib.dart" as tar2;
+import "dart:core" as core;
+
+import "dart:supported.by.spec";
+import "dart:unsupported.by.spec";
+import "dart:unsupported.by.target";
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_first;
+import "org-dartlang-testcase:///import_supported.by.target_lib.dart" as from_supported_by_target;
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_last;
+
+static method main() → dynamic {
+  spec::supportedBySpec();
+  by_::supportedByTarget();
+  spec2::unsupportedBySpec();
+  tar::unsupportedByTarget();
+  spe::unsupportedBySpecInternal();
+  self::expect("supported.by.spec", spe2::field);
+  self::expect("supported.by.target", tar2::field);
+  self::expect("supported.by.spec", spe2::field);
+  self::expect(true, #C1);
+  self::expect(true, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
+
+library dart.supported.by.spec /*isNonNullableByDefault*/;
+import self as spec;
+import "dart:_supported.by.target" as by_;
+additionalExports = (by_::supportedByTarget)
+
+export "dart:_supported.by.target";
+
+static method supportedBySpec() → void {}
+
+library dart.unsupported.by.spec /*isUnsupported,isNonNullableByDefault*/;
+import self as spec2;
+import "dart:_unsupported.by.spec_internal" as spe;
+additionalExports = (spe::unsupportedBySpecInternal)
+
+export "dart:_unsupported.by.spec_internal";
+
+static method unsupportedBySpec() → void {}
+
+library dart.unsupported.by.target /*isNonNullableByDefault*/;
+import self as tar;
+
+static method unsupportedByTarget() → void {}
+
+library /*isNonNullableByDefault*/;
+import self as spe2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.spec";
+
+library /*isNonNullableByDefault*/;
+import self as tar2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.target";
+
+library dart._supported.by_target /*isUnsupported,isNonNullableByDefault*/;
+import self as by_;
+
+static method supportedByTarget() → void {}
+
+library dart._unsupported.by.spec_internal /*isUnsupported,isNonNullableByDefault*/;
+import self as spe;
+
+static method unsupportedBySpecInternal() → void {}
+
+constants  {
+  #C1 = true
+  #C2 = false
+}
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.outline.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.outline.expect
new file mode 100644
index 00000000000..5e500abc406
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.outline.expect
@@ -0,0 +1,64 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+
+import "dart:supported.by.spec";
+import "dart:unsupported.by.spec";
+import "dart:unsupported.by.target";
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_first;
+import "org-dartlang-testcase:///import_supported.by.target_lib.dart" as from_supported_by_target;
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_last;
+
+static method main() → dynamic
+  ;
+static method expect(dynamic expected, dynamic actual) → dynamic
+  ;
+
+library dart.supported.by.spec /*isNonNullableByDefault*/;
+import self as self2;
+import "dart:_supported.by.target" as by_;
+additionalExports = (by_::supportedByTarget)
+
+export "dart:_supported.by.target";
+
+static method supportedBySpec() → void
+  ;
+
+library dart.unsupported.by.spec /*isUnsupported,isNonNullableByDefault*/;
+import self as self3;
+import "dart:_unsupported.by.spec_internal" as spe;
+additionalExports = (spe::unsupportedBySpecInternal)
+
+export "dart:_unsupported.by.spec_internal";
+
+static method unsupportedBySpec() → void
+  ;
+
+library dart.unsupported.by.target /*isNonNullableByDefault*/;
+import self as self4;
+
+static method unsupportedByTarget() → void
+  ;
+
+library /*isNonNullableByDefault*/;
+import self as self5;
+import "dart:core" as core;
+
+static field core::String field;
+
+library /*isNonNullableByDefault*/;
+import self as self6;
+import "dart:core" as core;
+
+static field core::String field;
+
+library dart._supported.by_target /*isUnsupported,isNonNullableByDefault*/;
+import self as by_;
+
+static method supportedByTarget() → void
+  ;
+
+library dart._unsupported.by.spec_internal /*isUnsupported,isNonNullableByDefault*/;
+import self as spe;
+
+static method unsupportedBySpecInternal() → void
+  ;
diff --git a/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.transformed.expect b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.transformed.expect
new file mode 100644
index 00000000000..85a3cb75f71
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/main.dart.weak.transformed.expect
@@ -0,0 +1,87 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:supported.by.spec" as spec;
+import "dart:_supported.by.target" as by_;
+import "dart:unsupported.by.spec" as spec2;
+import "dart:unsupported.by.target" as tar;
+import "dart:_unsupported.by.spec_internal" as spe;
+import "import_supported.by.spec_lib.dart" as spe2;
+import "import_supported.by.target_lib.dart" as tar2;
+import "dart:core" as core;
+
+import "dart:supported.by.spec";
+import "dart:unsupported.by.spec";
+import "dart:unsupported.by.target";
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_first;
+import "org-dartlang-testcase:///import_supported.by.target_lib.dart" as from_supported_by_target;
+import "org-dartlang-testcase:///import_supported.by.spec_lib.dart" as from_supported_by_spec_last;
+
+static method main() → dynamic {
+  spec::supportedBySpec();
+  by_::supportedByTarget();
+  spec2::unsupportedBySpec();
+  tar::unsupportedByTarget();
+  spe::unsupportedBySpecInternal();
+  self::expect("supported.by.spec", spe2::field);
+  self::expect("supported.by.target", tar2::field);
+  self::expect("supported.by.spec", spe2::field);
+  self::expect(true, #C1);
+  self::expect(true, #C1);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+  self::expect(false, #C2);
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
+
+library dart.supported.by.spec /*isNonNullableByDefault*/;
+import self as spec;
+import "dart:_supported.by.target" as by_;
+additionalExports = (by_::supportedByTarget)
+
+export "dart:_supported.by.target";
+
+static method supportedBySpec() → void {}
+
+library dart.unsupported.by.spec /*isUnsupported,isNonNullableByDefault*/;
+import self as spec2;
+import "dart:_unsupported.by.spec_internal" as spe;
+additionalExports = (spe::unsupportedBySpecInternal)
+
+export "dart:_unsupported.by.spec_internal";
+
+static method unsupportedBySpec() → void {}
+
+library dart.unsupported.by.target /*isNonNullableByDefault*/;
+import self as tar;
+
+static method unsupportedByTarget() → void {}
+
+library /*isNonNullableByDefault*/;
+import self as spe2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.spec";
+
+library /*isNonNullableByDefault*/;
+import self as tar2;
+import "dart:core" as core;
+
+static field core::String field = "supported.by.target";
+
+library dart._supported.by_target /*isUnsupported,isNonNullableByDefault*/;
+import self as by_;
+
+static method supportedByTarget() → void {}
+
+library dart._unsupported.by.spec_internal /*isUnsupported,isNonNullableByDefault*/;
+import self as spe;
+
+static method unsupportedBySpecInternal() → void {}
+
+constants  {
+  #C1 = true
+  #C2 = false
+}
diff --git a/pkg/front_end/testcases/general/supported_libraries/supported.by.spec_lib.dart b/pkg/front_end/testcases/general/supported_libraries/supported.by.spec_lib.dart
new file mode 100644
index 00000000000..ea5edc07d89
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/supported.by.spec_lib.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dart.supported.by.spec;
+
+export 'dart:_supported.by.target';
+
+void supportedBySpec() {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/supported.by.target_lib.dart b/pkg/front_end/testcases/general/supported_libraries/supported.by.target_lib.dart
new file mode 100644
index 00000000000..673f03dc8d6
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/supported.by.target_lib.dart
@@ -0,0 +1,7 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dart._supported.by_target;
+
+void supportedByTarget() {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_internal_lib.dart b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_internal_lib.dart
new file mode 100644
index 00000000000..7171f887735
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_internal_lib.dart
@@ -0,0 +1,7 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dart._unsupported.by.spec_internal;
+
+void unsupportedBySpecInternal() {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_lib.dart b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_lib.dart
new file mode 100644
index 00000000000..ac010bb4ed1
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.spec_lib.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dart.unsupported.by.spec;
+
+export 'dart:_unsupported.by.spec_internal';
+
+void unsupportedBySpec() {}
diff --git a/pkg/front_end/testcases/general/supported_libraries/unsupported.by.target_lib.dart b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.target_lib.dart
new file mode 100644
index 00000000000..0ef61dca381
--- /dev/null
+++ b/pkg/front_end/testcases/general/supported_libraries/unsupported.by.target_lib.dart
@@ -0,0 +1,7 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dart.unsupported.by.target;
+
+void unsupportedByTarget() {}
diff --git a/pkg/front_end/testcases/rasta/supports_reflection.dart b/pkg/front_end/testcases/rasta/supports_reflection.dart
index 4b53e520ae5..3aaf5eaa220 100644
--- a/pkg/front_end/testcases/rasta/supports_reflection.dart
+++ b/pkg/front_end/testcases/rasta/supports_reflection.dart
@@ -1,7 +1,9 @@
 // Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
+
 // @dart=2.9
+
 main() {
   print(const bool.fromEnvironment("dart.library.mirrors"));
 }
diff --git a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.expect b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.expect
index d2d46a3dfc5..362665c1467 100644
--- a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.expect
+++ b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.expect
@@ -7,5 +7,5 @@ static method main() → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
 }
diff --git a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.modular.expect b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.modular.expect
index d2d46a3dfc5..362665c1467 100644
--- a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.modular.expect
+++ b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.modular.expect
@@ -7,5 +7,5 @@ static method main() → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
 }
diff --git a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.transformed.expect b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.transformed.expect
index d2d46a3dfc5..362665c1467 100644
--- a/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/rasta/supports_reflection.dart.weak.transformed.expect
@@ -7,5 +7,5 @@ static method main() → dynamic {
 }
 
 constants  {
-  #C1 = false
+  #C1 = true
 }
diff --git a/pkg/front_end/testcases/text_serialization.status b/pkg/front_end/testcases/text_serialization.status
index 577ee4a3424..b66d7f8d262 100644
--- a/pkg/front_end/testcases/text_serialization.status
+++ b/pkg/front_end/testcases/text_serialization.status
@@ -33,7 +33,6 @@ general/bug30695: TypeCheckError
 general/bug31124: RuntimeError
 general/call: RuntimeError
 general/cascade: RuntimeError
-general/conditional_import: RuntimeError # Issue 47814
 general/constructor_initializer_invalid: RuntimeError
 general/covariant_field: TypeCheckError
 general/covariant_generic: RuntimeError
diff --git a/pkg/front_end/testcases/weak.status b/pkg/front_end/testcases/weak.status
index 699bd2923d2..8898c1a136c 100644
--- a/pkg/front_end/testcases/weak.status
+++ b/pkg/front_end/testcases/weak.status
@@ -47,7 +47,6 @@ general/bug30695: TypeCheckError
 general/bug31124: RuntimeError # Test has no main method (and we shouldn't add one).
 general/call: RuntimeError
 general/cascade: RuntimeError
-general/conditional_import: RuntimeError # Issue 47814
 general/constructor_initializer_invalid: RuntimeError # Fails execution after recovery
 general/covariant_field: TypeCheckError
 general/covariant_generic: RuntimeError
diff --git a/pkg/front_end/tool/dart_doctest_impl.dart b/pkg/front_end/tool/dart_doctest_impl.dart
index b03351a0a7b..c47c27ba17e 100644
--- a/pkg/front_end/tool/dart_doctest_impl.dart
+++ b/pkg/front_end/tool/dart_doctest_impl.dart
@@ -832,6 +832,7 @@ class DocTestIncrementalCompiler extends IncrementalCompiler {
       loader: loader,
       scope: libraryBuilder.scope.createNestedScope("dartdoctest"),
       nameOrigin: libraryBuilder,
+      isUnsupported: false,
     );
 
     if (libraryBuilder is DillLibraryBuilder) {
diff --git a/pkg/frontend_server/lib/frontend_server.dart b/pkg/frontend_server/lib/frontend_server.dart
index 756373624e7..c1dac119306 100644
--- a/pkg/frontend_server/lib/frontend_server.dart
+++ b/pkg/frontend_server/lib/frontend_server.dart
@@ -51,6 +51,10 @@ ArgParser argParser = ArgParser(allowTrailingOptions: true)
   ..addFlag('aot',
       help: 'Run compiler in AOT mode (enables whole-program transformations)',
       defaultsTo: false)
+  ..addFlag('support-mirrors',
+      help: 'Whether dart:mirrors is supported. By default dart:mirrors is '
+          'supported when --aot and --minimal-kernel are not used.',
+      defaultsTo: null)
   ..addFlag('tfa',
       help:
           'Enable global type flow analysis and related transformations in AOT mode.',
@@ -487,6 +491,18 @@ class FrontendCompiler implements CompilerInterface {
       }
     }
 
+    if (options['support-mirrors'] == true) {
+      if (options['aot']) {
+        print('Error: --support-mirrors option cannot be used with --aot');
+        return false;
+      }
+      if (options['minimal-kernel']) {
+        print('Error: --support-mirrors option cannot be used with '
+            '--minimal-kernel');
+        return false;
+      }
+    }
+
     if (options['incremental']) {
       if (options['from-dill'] != null) {
         print('Error: --from-dill option cannot be used with --incremental');
@@ -505,6 +521,8 @@ class FrontendCompiler implements CompilerInterface {
       options['target'],
       trackWidgetCreation: options['track-widget-creation'],
       nullSafety: compilerOptions.nnbdMode == NnbdMode.Strong,
+      supportMirrors: options['support-mirrors'] ??
+          !(options['aot'] || options['minimal-kernel']),
     );
     if (compilerOptions.target == null) {
       print('Failed to create front-end target ${options['target']}.');
diff --git a/pkg/kernel/binary.md b/pkg/kernel/binary.md
index b75aba6ee21..3df728f7a31 100644
--- a/pkg/kernel/binary.md
+++ b/pkg/kernel/binary.md
@@ -147,7 +147,7 @@ type CanonicalName {
 
 type ComponentFile {
   UInt32 magic = 0x90ABCDEF;
-  UInt32 formatVersion = 75;
+  UInt32 formatVersion = 76;
   Byte[10] shortSdkHash;
   List<String> problemsAsJson; // Described in problems.md.
   Library[] libraries;
@@ -237,7 +237,7 @@ type Name {
 }
 
 type Library {
-  Byte flags (isSynthetic, isNonNullableByDefault, nnbdModeBit1, nnbdModeBit2);
+  Byte flags (isSynthetic, isNonNullableByDefault, nnbdModeBit1, nnbdModeBit2, isUnsupported);
   UInt languageVersionMajor;
   UInt languageVersionMinor;
   CanonicalNameReference canonicalName;
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 99a2f861b27..51270c4c924 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -273,6 +273,7 @@ class Library extends NamedNode
   static const int NonNullableByDefaultFlag = 1 << 1;
   static const int NonNullableByDefaultModeBit1 = 1 << 2;
   static const int NonNullableByDefaultModeBit2 = 1 << 3;
+  static const int IsUnsupportedFlag = 1 << 4;
 
   int flags = 0;
 
@@ -322,6 +323,13 @@ class Library extends NamedNode
     }
   }
 
+  /// If true, the library is not supported through the 'dart.library.*' value
+  /// used in conditional imports and `bool.fromEnvironment` constants.
+  bool get isUnsupported => flags & IsUnsupportedFlag != 0;
+  void set isUnsupported(bool value) {
+    flags = value ? (flags | IsUnsupportedFlag) : (flags & ~IsUnsupportedFlag);
+  }
+
   String? name;
 
   /// Problems in this [Library] encoded as json objects.
diff --git a/pkg/kernel/lib/binary/tag.dart b/pkg/kernel/lib/binary/tag.dart
index ceaae4afa17..a8a29ff88a6 100644
--- a/pkg/kernel/lib/binary/tag.dart
+++ b/pkg/kernel/lib/binary/tag.dart
@@ -176,7 +176,7 @@ class Tag {
   /// Internal version of kernel binary format.
   /// Bump it when making incompatible changes in kernel binaries.
   /// Keep in sync with runtime/vm/kernel_binary.h, pkg/kernel/binary.md.
-  static const int BinaryFormatVersion = 75;
+  static const int BinaryFormatVersion = 76;
 }
 
 abstract class ConstantTag {
diff --git a/pkg/kernel/lib/target/targets.dart b/pkg/kernel/lib/target/targets.dart
index 804f26aa3c5..173d367f636 100644
--- a/pkg/kernel/lib/target/targets.dart
+++ b/pkg/kernel/lib/target/targets.dart
@@ -15,16 +15,20 @@ final List<String> targetNames = targets.keys.toList();
 class TargetFlags {
   final bool trackWidgetCreation;
   final bool enableNullSafety;
+  final bool supportMirrors;
 
   const TargetFlags(
-      {this.trackWidgetCreation = false, this.enableNullSafety = false});
+      {this.trackWidgetCreation = false,
+      this.enableNullSafety = false,
+      this.supportMirrors = true});
 
   @override
   bool operator ==(other) {
     if (identical(this, other)) return true;
     return other is TargetFlags &&
         trackWidgetCreation == other.trackWidgetCreation &&
-        enableNullSafety == other.enableNullSafety;
+        enableNullSafety == other.enableNullSafety &&
+        supportMirrors == other.supportMirrors;
   }
 
   @override
@@ -32,6 +36,7 @@ class TargetFlags {
     int hash = 485786;
     hash = 0x3fffffff & (hash * 31 + (hash ^ trackWidgetCreation.hashCode));
     hash = 0x3fffffff & (hash * 31 + (hash ^ enableNullSafety.hashCode));
+    hash = 0x3fffffff & (hash * 31 + (hash ^ supportMirrors.hashCode));
     return hash;
   }
 }
@@ -151,6 +156,95 @@ class ConstantsBackend {
   bool get keepLocals => false;
 }
 
+/// Interface used for determining whether a `dart:*` is considered supported
+/// for the current target.
+abstract class DartLibrarySupport {
+  /// Returns `true` if the 'dart:[libraryName]' library is supported.
+  ///
+  /// [isSupportedBySpec] is `true` if the dart library was supported by the
+  /// libraries specification.
+  ///
+  /// This is used to allow AOT to consider `dart:mirrors` as unsupported
+  /// despite it being supported in the platform dill, and dart2js to consider
+  /// `dart:_dart2js_runtime_metrics` to be supported despite it being an
+  /// internal library.
+  bool computeDartLibrarySupport(String libraryName,
+      {required bool isSupportedBySpec});
+
+  static const String dartLibraryPrefix = "dart.library.";
+
+  static bool isDartLibraryQualifier(String dottedName) {
+    return dottedName.startsWith(dartLibraryPrefix);
+  }
+
+  static String getDartLibraryName(String dottedName) {
+    assert(isDartLibraryQualifier(dottedName));
+    return dottedName.substring(dartLibraryPrefix.length);
+  }
+
+  /// Returns `"true"` if the "dart:[libraryName]" is supported and `""`
+  /// otherwise.
+  ///
+  /// This is used to determine conditional imports and `bool.fromEnvironment`
+  /// constant values for "dart.library.[libraryName]" values.
+  static String getDartLibrarySupportValue(String libraryName,
+      {required bool libraryExists,
+      required bool isSynthetic,
+      required bool isUnsupported,
+      required DartLibrarySupport dartLibrarySupport}) {
+    // A `dart:` library can be unsupported for several reasons:
+    // * If the library doesn't exist from source or from dill, it is not
+    //   supported.
+    // * If the library has been synthesized, then it doesn't exist, but has
+    //   been synthetically created due to an explicit import or export of it,
+    //   in which case it is also not supported.
+    // * If the library is marked as not supported in the libraries
+    //   specification, it does exist, but is not supported.
+    // * If the library is marked as supported in the libraries specification,
+    //   it does exist and is potentially supported. Still the [Target] can
+    //   consider it unsupported. This is for instance used to consider
+    //   `dart:mirrors` as unsupported in AOT. The platform dill is shared with
+    //   JIT, so the library exists and is marked as supported, but for AOT
+    //   compilation it is still unsupported.
+    bool isSupported = libraryExists && !isSynthetic && !isUnsupported;
+    isSupported = dartLibrarySupport.computeDartLibrarySupport(libraryName,
+        isSupportedBySpec: isSupported);
+    return isSupported ? "true" : "";
+  }
+}
+
+/// [DartLibrarySupport] that only relies on the "supported" property of
+/// the libraries specification.
+class DefaultDartLibrarySupport implements DartLibrarySupport {
+  const DefaultDartLibrarySupport();
+
+  @override
+  bool computeDartLibrarySupport(String libraryName,
+          {required bool isSupportedBySpec}) =>
+      isSupportedBySpec;
+}
+
+/// [DartLibrarySupport] that supports overriding `dart:*` library support
+/// otherwise defined by the libraries specification.
+class CustomizedDartLibrarySupport implements DartLibrarySupport {
+  final Set<String> supported;
+  final Set<String> unsupported;
+
+  const CustomizedDartLibrarySupport(
+      {this.supported: const {}, this.unsupported: const {}});
+
+  @override
+  bool computeDartLibrarySupport(String libraryName,
+      {required bool isSupportedBySpec}) {
+    if (supported.contains(libraryName)) {
+      return true;
+    } else if (unsupported.contains(libraryName)) {
+      return false;
+    }
+    return isSupportedBySpec;
+  }
+}
+
 /// A target provides backend-specific options for generating kernel IR.
 abstract class Target {
   TargetFlags get flags;
@@ -434,7 +528,7 @@ abstract class Target {
   /// Returns the configured component.
   Component configureComponent(Component component) => component;
 
-  // Configure environment defines in a target-specific way.
+  /// Configure environment defines in a target-specific way.
   Map<String, String> updateEnvironmentDefines(Map<String, String> map) => map;
 
   @override
@@ -453,6 +547,16 @@ abstract class Target {
   Class? concreteStringLiteralClass(CoreTypes coreTypes, String value) => null;
 
   ConstantsBackend get constantsBackend;
+
+  /// Returns an [DartLibrarySupport] the defines which, if any, of the
+  /// `dart:` libraries supported in the platform, that should not be
+  /// considered supported when queried in conditional imports and
+  /// `bool.fromEnvironment` constants.
+  ///
+  /// This is used treat `dart:mirrors` as unsupported in AOT but supported
+  /// in JIT.
+  DartLibrarySupport get dartLibrarySupport =>
+      const DefaultDartLibrarySupport();
 }
 
 class NoneConstantsBackend extends ConstantsBackend {
@@ -657,6 +761,8 @@ class TestTargetFlags extends TargetFlags {
   final bool? forceStaticFieldLoweringForTesting;
   final bool? forceNoExplicitGetterCallsForTesting;
   final int? forceConstructorTearOffLoweringForTesting;
+  final Set<String> supportedDartLibraries;
+  final Set<String> unsupportedDartLibraries;
 
   const TestTargetFlags(
       {bool trackWidgetCreation = false,
@@ -665,7 +771,9 @@ class TestTargetFlags extends TargetFlags {
       this.forceStaticFieldLoweringForTesting,
       this.forceNoExplicitGetterCallsForTesting,
       this.forceConstructorTearOffLoweringForTesting,
-      bool enableNullSafety = false})
+      bool enableNullSafety = false,
+      this.supportedDartLibraries: const {},
+      this.unsupportedDartLibraries: const {}})
       : super(
             trackWidgetCreation: trackWidgetCreation,
             enableNullSafety: enableNullSafety);
@@ -698,6 +806,29 @@ mixin TestTargetMixin on Target {
   int get enabledConstructorTearOffLowerings =>
       flags.forceConstructorTearOffLoweringForTesting ??
       super.enabledConstructorTearOffLowerings;
+
+  @override
+  late final DartLibrarySupport dartLibrarySupport =
+      new TestDartLibrarySupport(super.dartLibrarySupport, flags);
+}
+
+class TestDartLibrarySupport implements DartLibrarySupport {
+  final DartLibrarySupport delegate;
+  final TestTargetFlags flags;
+
+  TestDartLibrarySupport(this.delegate, this.flags);
+
+  @override
+  bool computeDartLibrarySupport(String libraryName,
+      {required bool isSupportedBySpec}) {
+    if (flags.supportedDartLibraries.contains(libraryName)) {
+      return true;
+    } else if (flags.unsupportedDartLibraries.contains(libraryName)) {
+      return false;
+    }
+    return delegate.computeDartLibrarySupport(libraryName,
+        isSupportedBySpec: isSupportedBySpec);
+  }
 }
 
 class TargetWrapper extends Target {
diff --git a/pkg/kernel/lib/text/ast_to_text.dart b/pkg/kernel/lib/text/ast_to_text.dart
index 4f718b930dd..7f0ce5d494a 100644
--- a/pkg/kernel/lib/text/ast_to_text.dart
+++ b/pkg/kernel/lib/text/ast_to_text.dart
@@ -454,8 +454,15 @@ class Printer extends Visitor<void> with VisitorVoidMixin {
     if (name != null) {
       writeWord(name);
     }
+    List<String> flags = [];
+    if (library.isUnsupported) {
+      flags.add('isUnsupported');
+    }
     if (library.isNonNullableByDefault) {
-      writeWord("/*isNonNullableByDefault*/");
+      flags.add('isNonNullableByDefault');
+    }
+    if (flags.isNotEmpty) {
+      writeWord('/*${flags.join(',')}*/');
     }
     endLine(';');
 
diff --git a/pkg/vm/lib/kernel_front_end.dart b/pkg/vm/lib/kernel_front_end.dart
index 871ee7deb17..74c8888b2e6 100644
--- a/pkg/vm/lib/kernel_front_end.dart
+++ b/pkg/vm/lib/kernel_front_end.dart
@@ -77,6 +77,10 @@ void declareCompilerOptions(ArgParser args) {
       help:
           'Produce kernel file for AOT compilation (enables global transformations).',
       defaultsTo: false);
+  args.addFlag('support-mirrors',
+      help: 'Whether dart:mirrors is supported. By default dart:mirrors is '
+          'supported when --aot and --minimal-kernel are not used.',
+      defaultsTo: null);
   args.addOption('depfile', help: 'Path to output Ninja depfile');
   args.addOption('from-dill',
       help: 'Read existing dill file instead of compiling from sources',
@@ -193,6 +197,7 @@ Future<int> runCompiler(ArgResults options, String usage) async {
   final bool splitOutputByPackages = options['split-output-by-packages'];
   final String? manifestFilename = options['manifest'];
   final String? dataDir = options['component-name'] ?? options['data-dir'];
+  final bool? supportMirrors = options['support-mirrors'];
 
   final bool minimalKernel = options['minimal-kernel'];
   final bool treeShakeWriteOnlyFields = options['tree-shake-write-only-fields'];
@@ -215,6 +220,18 @@ Future<int> runCompiler(ArgResults options, String usage) async {
     }
   }
 
+  if (supportMirrors == true) {
+    if (aot) {
+      print('Error: --support-mirrors option cannot be used with --aot');
+      return badUsageExitCode;
+    }
+    if (minimalKernel) {
+      print('Error: --support-mirrors option cannot be used with '
+          '--minimal-kernel');
+      return badUsageExitCode;
+    }
+  }
+
   final fileSystem =
       createFrontEndFileSystem(fileSystemScheme, fileSystemRoots);
 
@@ -258,11 +275,10 @@ Future<int> runCompiler(ArgResults options, String usage) async {
     await autoDetectNullSafetyMode(mainUri, compilerOptions);
   }
 
-  compilerOptions.target = createFrontEndTarget(
-    targetName,
-    trackWidgetCreation: options['track-widget-creation'],
-    nullSafety: compilerOptions.nnbdMode == NnbdMode.Strong,
-  );
+  compilerOptions.target = createFrontEndTarget(targetName,
+      trackWidgetCreation: options['track-widget-creation'],
+      nullSafety: compilerOptions.nnbdMode == NnbdMode.Strong,
+      supportMirrors: supportMirrors ?? !(aot || minimalKernel));
   if (compilerOptions.target == null) {
     print('Failed to create front-end target $targetName.');
     return badUsageExitCode;
@@ -442,6 +458,7 @@ Future runGlobalTransformations(
     {bool minimalKernel: false,
     bool treeShakeWriteOnlyFields: false,
     bool useRapidTypeAnalysis: true}) async {
+  assert(!target.flags.supportMirrors);
   if (errorDetector.hasCompilationErrors) return;
 
   final coreTypes = new CoreTypes(component);
@@ -585,12 +602,16 @@ Future<void> autoDetectNullSafetyMode(
 
 /// Create front-end target with given name.
 Target? createFrontEndTarget(String targetName,
-    {bool trackWidgetCreation = false, bool nullSafety = false}) {
+    {bool trackWidgetCreation = false,
+    bool nullSafety = false,
+    bool supportMirrors = true}) {
   // Make sure VM-specific targets are available.
   installAdditionalTargets();
 
   final TargetFlags targetFlags = new TargetFlags(
-      trackWidgetCreation: trackWidgetCreation, enableNullSafety: nullSafety);
+      trackWidgetCreation: trackWidgetCreation,
+      enableNullSafety: nullSafety,
+      supportMirrors: supportMirrors);
   return getTarget(targetName, targetFlags);
 }
 
diff --git a/pkg/vm/lib/target/vm.dart b/pkg/vm/lib/target/vm.dart
index cb866c85543..da77a9ec48f 100644
--- a/pkg/vm/lib/target/vm.dart
+++ b/pkg/vm/lib/target/vm.dart
@@ -500,18 +500,11 @@ class VmTarget extends Target {
     // TODO(alexmarkov): Call this from the front-end in order to have
     //  the same defines when compiling platform.
     map['dart.isVM'] = 'true';
-    // TODO(dartbug.com/36460): Derive dart.library.* definitions from platform.
-    for (String library in extraRequiredLibraries) {
-      Uri libraryUri = Uri.parse(library);
-      if (libraryUri.scheme == 'dart') {
-        final path = libraryUri.path;
-        if (!path.startsWith('_')) {
-          map['dart.library.${path}'] = 'true';
-        }
-      }
-    }
-    // dart:core is not mentioned in Target.extraRequiredLibraries.
-    map['dart.library.core'] = 'true';
     return map;
   }
+
+  @override
+  DartLibrarySupport get dartLibrarySupport => flags.supportMirrors
+      ? const DefaultDartLibrarySupport()
+      : const CustomizedDartLibrarySupport(unsupported: {'mirrors'});
 }
diff --git a/pkg/vm/test/modular_kernel_plus_aot_test.dart b/pkg/vm/test/modular_kernel_plus_aot_test.dart
index 090a08dec68..8e05c6c71db 100644
--- a/pkg/vm/test/modular_kernel_plus_aot_test.dart
+++ b/pkg/vm/test/modular_kernel_plus_aot_test.dart
@@ -28,7 +28,7 @@ main() async {
   final Uri packagesFile = sdkRootFile('.packages');
   final Uri librariesFile = sdkRootFile('sdk/lib/libraries.json');
 
-  final vmTarget = VmTarget(TargetFlags());
+  final vmTarget = VmTarget(TargetFlags(supportMirrors: false));
 
   await withTempDirectory((Uri uri) async {
     final mixinFilename = uri.resolve('mixin.dart');
diff --git a/pkg/vm/test/transformations/protobuf_aware_treeshaker/treeshaker_test.dart b/pkg/vm/test/transformations/protobuf_aware_treeshaker/treeshaker_test.dart
index 4febdf91c0f..cf616edd423 100644
--- a/pkg/vm/test/transformations/protobuf_aware_treeshaker/treeshaker_test.dart
+++ b/pkg/vm/test/transformations/protobuf_aware_treeshaker/treeshaker_test.dart
@@ -71,7 +71,7 @@ Future<void> shakeAndRun(Uri source) async {
 }
 
 Future<void> compileAOT(Uri source) async {
-  final target = TestingVmTarget(TargetFlags());
+  final target = TestingVmTarget(TargetFlags(supportMirrors: false));
   Component component =
       await compileTestCaseToKernelProgram(source, target: target);
 
diff --git a/runtime/vm/compiler/frontend/kernel_translation_helper.h b/runtime/vm/compiler/frontend/kernel_translation_helper.h
index ea30fa5c184..315e4689352 100644
--- a/runtime/vm/compiler/frontend/kernel_translation_helper.h
+++ b/runtime/vm/compiler/frontend/kernel_translation_helper.h
@@ -817,6 +817,7 @@ class LibraryHelper {
     kIsNonNullableByDefault = 1 << 1,
     kNonNullableByDefaultCompiledModeBit1 = 1 << 2,
     kNonNullableByDefaultCompiledModeBit2 = 1 << 3,
+    kUnsupported = 1 << 4,
   };
 
   explicit LibraryHelper(KernelReaderHelper* helper, uint32_t binary_version)
diff --git a/runtime/vm/kernel_binary.h b/runtime/vm/kernel_binary.h
index 891a2b72841..ff0cfa2be06 100644
--- a/runtime/vm/kernel_binary.h
+++ b/runtime/vm/kernel_binary.h
@@ -20,8 +20,8 @@ namespace kernel {
 static const uint32_t kMagicProgramFile = 0x90ABCDEFu;
 
 // Both version numbers are inclusive.
-static const uint32_t kMinSupportedKernelFormatVersion = 75;
-static const uint32_t kMaxSupportedKernelFormatVersion = 75;
+static const uint32_t kMinSupportedKernelFormatVersion = 76;
+static const uint32_t kMaxSupportedKernelFormatVersion = 76;
 
 // Keep in sync with package:kernel/lib/binary/tag.dart
 #define KERNEL_TAG_LIST(V)                                                     \