diff --git a/.dart_tool/package_config.json b/.dart_tool/package_config.json
index b6f9a73374c..3857a2df779 100644
--- a/.dart_tool/package_config.json
+++ b/.dart_tool/package_config.json
@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-09-08T14:55:24.803714",
+  "generated": "2021-09-10T10:06:41.987732",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -494,7 +494,7 @@
       "name": "observatory_2",
       "rootUri": "../runtime/observatory_2",
       "packageUri": "lib/",
-      "languageVersion": "2.2"
+      "languageVersion": "2.6"
     },
     {
       "name": "observatory_test_package",
diff --git a/pkg/dev_compiler/lib/src/kernel/expression_compiler.dart b/pkg/dev_compiler/lib/src/kernel/expression_compiler.dart
index 23cc5bb8d68..8d195b37696 100644
--- a/pkg/dev_compiler/lib/src/kernel/expression_compiler.dart
+++ b/pkg/dev_compiler/lib/src/kernel/expression_compiler.dart
@@ -440,8 +440,8 @@ class ExpressionCompiler {
         scope.typeParameters,
         debugProcedureName,
         scope.library.importUri,
-        scope.cls?.name,
-        scope.isStatic);
+        className: scope.cls?.name,
+        isStatic: scope.isStatic);
 
     _log('Compiled expression to kernel');
 
diff --git a/pkg/front_end/lib/src/api_prototype/incremental_kernel_generator.dart b/pkg/front_end/lib/src/api_prototype/incremental_kernel_generator.dart
index a6d071f35cf..d28d71c6058 100644
--- a/pkg/front_end/lib/src/api_prototype/incremental_kernel_generator.dart
+++ b/pkg/front_end/lib/src/api_prototype/incremental_kernel_generator.dart
@@ -136,8 +136,9 @@ abstract class IncrementalKernelGenerator {
       List<TypeParameter> typeDefinitions,
       String syntheticProcedureName,
       Uri libraryUri,
-      [String? className,
-      bool isStatic = false]);
+      {String? className,
+      String? methodName,
+      bool isStatic = false});
 
   /// Sets experimental features.
   ///
diff --git a/pkg/front_end/lib/src/api_prototype/lowering_predicates.dart b/pkg/front_end/lib/src/api_prototype/lowering_predicates.dart
index d1a2a4e9b3b..1375e06a6c4 100644
--- a/pkg/front_end/lib/src/api_prototype/lowering_predicates.dart
+++ b/pkg/front_end/lib/src/api_prototype/lowering_predicates.dart
@@ -660,8 +660,8 @@ String extractLocalNameFromLateLoweredSetter(String name) {
 ///
 /// where '#this' is the synthetic "extension this" parameter.
 bool isExtensionThis(VariableDeclaration node) {
-  assert(
-      node.isLowered || node.name == null || !isExtensionThisName(node.name));
+  assert(node.isLowered || node.name == null || !isExtensionThisName(node.name),
+      "$node has name ${node.name} and node.isLowered = ${node.isLowered}");
   return node.isLowered && isExtensionThisName(node.name);
 }
 
diff --git a/pkg/front_end/lib/src/api_unstable/vm.dart b/pkg/front_end/lib/src/api_unstable/vm.dart
index f592c51077c..e981bd36384 100644
--- a/pkg/front_end/lib/src/api_unstable/vm.dart
+++ b/pkg/front_end/lib/src/api_unstable/vm.dart
@@ -29,6 +29,8 @@ export '../api_prototype/incremental_kernel_generator.dart'
 export '../api_prototype/kernel_generator.dart'
     show kernelForModule, kernelForProgram;
 
+export '../api_prototype/lowering_predicates.dart' show isExtensionThisName;
+
 export '../api_prototype/memory_file_system.dart' show MemoryFileSystem;
 
 export '../api_prototype/standard_file_system.dart' show StandardFileSystem;
diff --git a/pkg/front_end/lib/src/fasta/incremental_compiler.dart b/pkg/front_end/lib/src/fasta/incremental_compiler.dart
index ef7f4618ac1..d3a46e75ebb 100644
--- a/pkg/front_end/lib/src/fasta/incremental_compiler.dart
+++ b/pkg/front_end/lib/src/fasta/incremental_compiler.dart
@@ -11,7 +11,15 @@ import 'package:_fe_analyzer_shared/src/scanner/abstract_scanner.dart'
 
 import 'package:front_end/src/api_prototype/experimental_flags.dart';
 import 'package:front_end/src/api_prototype/front_end.dart';
+
+import 'package:front_end/src/api_prototype/lowering_predicates.dart'
+    show isExtensionThisName;
+
 import 'package:front_end/src/base/nnbd_mode.dart';
+
+import 'package:front_end/src/fasta/builder/member_builder.dart'
+    show MemberBuilder;
+
 import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:front_end/src/fasta/source/source_loader.dart';
 import 'package:kernel/binary/ast_from_binary.dart'
@@ -34,6 +42,7 @@ import 'package:kernel/kernel.dart'
         Component,
         DartType,
         Expression,
+        Extension,
         FunctionNode,
         Library,
         LibraryDependency,
@@ -48,7 +57,8 @@ import 'package:kernel/kernel.dart'
         Source,
         Supertype,
         TreeNode,
-        TypeParameter;
+        TypeParameter,
+        VariableDeclaration;
 
 import 'package:kernel/canonical_name.dart'
     show CanonicalNameError, CanonicalNameSdkError;
@@ -71,6 +81,8 @@ import 'builder/builder.dart' show Builder;
 
 import 'builder/class_builder.dart' show ClassBuilder;
 
+import 'builder/extension_builder.dart' show ExtensionBuilder;
+
 import 'builder/field_builder.dart' show FieldBuilder;
 
 import 'builder/library_builder.dart' show LibraryBuilder;
@@ -1907,8 +1919,9 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
       List<TypeParameter> typeDefinitions,
       String syntheticProcedureName,
       Uri libraryUri,
-      [String? className,
-      bool isStatic = false]) async {
+      {String? className,
+      String? methodName,
+      bool isStatic = false}) async {
     assert(dillLoadedData != null && userCode != null);
 
     return await context.runInContext((_) async {
@@ -1923,6 +1936,26 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
         cls = classBuilder?.cls;
         if (cls == null) return null;
       }
+      Extension? extension;
+      String? extensionName;
+      if (methodName != null) {
+        int indexOfDot = methodName.indexOf(".");
+        if (indexOfDot >= 0) {
+          String beforeDot = methodName.substring(0, indexOfDot);
+          String afterDot = methodName.substring(indexOfDot + 1);
+          Builder? builder = libraryBuilder.scopeBuilder[beforeDot];
+          extensionName = beforeDot;
+          if (builder is ExtensionBuilder) {
+            extension = builder.extension;
+            Builder? subBuilder = builder.scopeBuilder[afterDot];
+            if (subBuilder is MemberBuilder) {
+              if (subBuilder.isExtensionInstanceMember) {
+                isStatic = false;
+              }
+            }
+          }
+        }
+      }
 
       userCode!.loader.resetSeenMessages();
 
@@ -1937,8 +1970,14 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
           return null;
         }
       }
+      int index = 0;
       for (String name in definitions.keys) {
-        if (!isLegalIdentifier(name)) {
+        index++;
+        if (!(isLegalIdentifier(name) ||
+            (extension != null &&
+                !isStatic &&
+                index == 1 &&
+                isExtensionThisName(name)))) {
           userCode!.loader.addProblem(
               templateIncrementalCompilerIllegalParameter.withArguments(name),
               // TODO: pass variable declarations instead of
@@ -2009,13 +2048,29 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
           positionalParameters: definitions.keys
               .map((name) =>
                   new VariableDeclarationImpl(name, 0, type: definitions[name])
-                    ..fileOffset =
-                        cls?.fileOffset ?? libraryBuilder.library.fileOffset)
+                    ..fileOffset = cls?.fileOffset ??
+                        extension?.fileOffset ??
+                        libraryBuilder.library.fileOffset)
               .toList());
 
+      VariableDeclaration? extensionThis;
+      if (extension != null &&
+          !isStatic &&
+          parameters.positionalParameters.isNotEmpty) {
+        // We expect the first parameter to be called #this and be special.
+        if (isExtensionThisName(parameters.positionalParameters.first.name)) {
+          extensionThis = parameters.positionalParameters.first;
+          extensionThis.isLowered = true;
+        }
+      }
+
       debugLibrary.build(userCode!.loader.coreLibrary, modifyTarget: false);
       Expression compiledExpression = await userCode!.loader.buildExpression(
-          debugLibrary, className, className != null && !isStatic, parameters);
+          debugLibrary,
+          className ?? extensionName,
+          (className != null && !isStatic) || extensionThis != null,
+          parameters,
+          extensionThis);
 
       Procedure procedure = new Procedure(
           new Name(syntheticProcedureName), ProcedureKind.Method, parameters,
@@ -2026,7 +2081,7 @@ class IncrementalCompiler implements IncrementalKernelGenerator {
         ..parent = parameters;
 
       procedure.fileUri = debugLibrary.fileUri;
-      procedure.parent = className != null ? cls : libraryBuilder.library;
+      procedure.parent = cls ?? libraryBuilder.library;
 
       userCode!.uriToSource.remove(debugExprUri);
       userCode!.loader.sourceBytes.remove(debugExprUri);
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 110567c0a8f..35bf322aee0 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -45,6 +45,7 @@ import 'package:kernel/ast.dart'
         Reference,
         Supertype,
         TreeNode,
+        VariableDeclaration,
         Version;
 
 import 'package:kernel/class_hierarchy.dart'
@@ -513,26 +514,32 @@ class SourceLoader extends Loader {
     }
   }
 
-  // TODO(johnniwinther,jensj): Handle expression in extensions?
   Future<Expression> buildExpression(
       SourceLibraryBuilder libraryBuilder,
-      String? enclosingClass,
+      String? enclosingClassOrExtension,
       bool isClassInstanceMember,
-      FunctionNode parameters) async {
+      FunctionNode parameters,
+      VariableDeclaration? extensionThis) async {
     Token token = await tokenize(libraryBuilder, suppressLexicalErrors: false);
     DietListener dietListener = createDietListener(libraryBuilder);
 
     Builder parent = libraryBuilder;
-    if (enclosingClass != null) {
+    if (enclosingClassOrExtension != null) {
       Builder? cls = dietListener.memberScope
-          .lookup(enclosingClass, -1, libraryBuilder.fileUri);
+          .lookup(enclosingClassOrExtension, -1, libraryBuilder.fileUri);
       if (cls is ClassBuilder) {
         parent = cls;
         dietListener
           ..currentDeclaration = cls
           ..memberScope = cls.scope.copyWithParent(
               dietListener.memberScope.withTypeVariables(cls.typeVariables),
-              "debugExpression in $enclosingClass");
+              "debugExpression in class $enclosingClassOrExtension");
+      } else if (cls is ExtensionBuilder) {
+        parent = cls;
+        dietListener
+          ..currentDeclaration = cls
+          ..memberScope = cls.scope.copyWithParent(dietListener.memberScope,
+              "debugExpression in extension $enclosingClassOrExtension");
       }
     }
     ProcedureBuilder builder = new SourceProcedureBuilder(
@@ -562,7 +569,8 @@ class SourceLoader extends Loader {
       ..parent = parent;
     BodyBuilder listener = dietListener.createListener(
         builder, dietListener.memberScope,
-        isDeclarationInstanceMember: isClassInstanceMember);
+        isDeclarationInstanceMember: isClassInstanceMember,
+        extensionThis: extensionThis);
 
     return listener.parseSingleExpression(
         new Parser(listener,
diff --git a/pkg/front_end/test/fasta/expression_suite.dart b/pkg/front_end/test/fasta/expression_suite.dart
index 94cef6eedc2..793aeb0779f 100644
--- a/pkg/front_end/test/fasta/expression_suite.dart
+++ b/pkg/front_end/test/fasta/expression_suite.dart
@@ -140,6 +140,8 @@ class TestCase {
 
   final String className;
 
+  final String methodName;
+
   String expression;
 
   List<CompilationResult> results = [];
@@ -153,6 +155,7 @@ class TestCase {
       this.isStaticMethod,
       this.library,
       this.className,
+      this.methodName,
       this.expression);
 
   @override
@@ -260,6 +263,7 @@ class ReadTest extends Step<TestDescription, List<TestCase>, Context> {
     bool isStaticMethod = false;
     Uri library;
     String className;
+    String methodName;
     String expression;
 
     dynamic maps = loadYamlNode(contents, sourceUrl: uri);
@@ -281,6 +285,8 @@ class ReadTest extends Step<TestDescription, List<TestCase>, Context> {
           if (uri.fragment != null && uri.fragment != '') {
             className = uri.fragment;
           }
+        } else if (key == "method") {
+          methodName = value as String;
         } else if (key == "definitions") {
           definitions = (value as YamlList).map((x) => x as String).toList();
         } else if (key == "type_definitions") {
@@ -292,8 +298,17 @@ class ReadTest extends Step<TestDescription, List<TestCase>, Context> {
           expression = value;
         }
       }
-      var test = new TestCase(description, entryPoint, import, definitions,
-          typeDefinitions, isStaticMethod, library, className, expression);
+      var test = new TestCase(
+          description,
+          entryPoint,
+          import,
+          definitions,
+          typeDefinitions,
+          isStaticMethod,
+          library,
+          className,
+          methodName,
+          expression);
       var result = test.validate();
       if (result != null) {
         return new Result.fail(tests, result);
@@ -325,13 +340,15 @@ class CompileExpression extends Step<List<TestCase>, List<TestCase>, Context> {
     }
 
     Procedure compiledProcedure = await compiler.compileExpression(
-        test.expression,
-        definitions,
-        typeParams,
-        "debugExpr",
-        test.library,
-        test.className,
-        test.isStaticMethod);
+      test.expression,
+      definitions,
+      typeParams,
+      "debugExpr",
+      test.library,
+      className: test.className,
+      methodName: test.methodName,
+      isStatic: test.isStaticMethod,
+    );
     List<DiagnosticMessage> errors = context.takeErrors();
     test.results.add(new CompilationResult(compiledProcedure, errors));
     if (compiledProcedure != null) {
diff --git a/pkg/front_end/testcases/expression/extension_this.expression.yaml b/pkg/front_end/testcases/expression/extension_this.expression.yaml
new file mode 100644
index 00000000000..3ac2b8f2ca5
--- /dev/null
+++ b/pkg/front_end/testcases/expression/extension_this.expression.yaml
@@ -0,0 +1,10 @@
+# Copyright (c) 2021, 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.
+
+entry_point: "main.dart"
+definitions: ["#this"]
+position: "main.dart"
+method: "Foo.parseAsInt"
+expression: |
+  () { print(getFortyTwo()); return this; }
diff --git a/pkg/front_end/testcases/expression/extension_this.expression.yaml.expect b/pkg/front_end/testcases/expression/extension_this.expression.yaml.expect
new file mode 100644
index 00000000000..aed2ac61e1b
--- /dev/null
+++ b/pkg/front_end/testcases/expression/extension_this.expression.yaml.expect
@@ -0,0 +1,7 @@
+Errors: {
+}
+method /* from org-dartlang-debug:synthetic_debug_expression */ debugExpr(lowered dynamic #this) → dynamic
+  return () → dynamic {
+    dart.core::print(main::Foo|getFortyTwo(#this as{TypeError,ForDynamic} dart.core::String*));
+    return #this;
+  };
diff --git a/pkg/front_end/testcases/expression/main.dart b/pkg/front_end/testcases/expression/main.dart
index a4fefb51d0e..f65db931d22 100644
--- a/pkg/front_end/testcases/expression/main.dart
+++ b/pkg/front_end/testcases/expression/main.dart
@@ -79,3 +79,16 @@ class MiddlewareApi<State extends Built<State, StateBuilder>,
 main() {
   exit(0);
 }
+
+extension Foo on String {
+  int parseAsInt() {
+    int result = int.parse(this);
+    print("Parsed $this to $result");
+    print(getFortyTwo());
+    return result;
+  }
+
+  int getFortyTwo() {
+    return 42;
+  }
+}
diff --git a/pkg/frontend_server/lib/frontend_server.dart b/pkg/frontend_server/lib/frontend_server.dart
index 29589fc317b..9545afb69cd 100644
--- a/pkg/frontend_server/lib/frontend_server.dart
+++ b/pkg/frontend_server/lib/frontend_server.dart
@@ -274,6 +274,7 @@ abstract class CompilerInterface {
       List<String> typeDefinitions,
       String libraryUri,
       String klass,
+      String method,
       bool isStatic);
 
   /// Compiles [expression] in [libraryUri] at [line]:[column] to JavaScript
@@ -806,11 +807,12 @@ class FrontendCompiler implements CompilerInterface {
       List<String> typeDefinitions,
       String libraryUri,
       String klass,
+      String method,
       bool isStatic) async {
     final String boundaryKey = Uuid().generateV4();
     _outputStream.writeln('result $boundaryKey');
-    Procedure procedure = await _generator.compileExpression(
-        expression, definitions, typeDefinitions, libraryUri, klass, isStatic);
+    Procedure procedure = await _generator.compileExpression(expression,
+        definitions, typeDefinitions, libraryUri, klass, method, isStatic);
     if (procedure != null) {
       Component component = createExpressionEvaluationComponent(procedure);
       final IOSink sink = File(_kernelBinaryFilename).openWrite();
@@ -1091,6 +1093,7 @@ class _CompileExpressionRequest {
   List<String> typeDefs = <String>[];
   String library;
   String klass;
+  String method;
   bool isStatic;
 }
 
@@ -1231,6 +1234,7 @@ StreamSubscription<String> listenAndCompile(CompilerInterface compiler,
               compileExpressionRequest.typeDefs,
               compileExpressionRequest.library,
               compileExpressionRequest.klass,
+              compileExpressionRequest.method,
               compileExpressionRequest.isStatic);
         } else {
           compiler
diff --git a/pkg/vm/bin/kernel_service.dart b/pkg/vm/bin/kernel_service.dart
index e3a221e59b3..88549103878 100644
--- a/pkg/vm/bin/kernel_service.dart
+++ b/pkg/vm/bin/kernel_service.dart
@@ -501,12 +501,13 @@ Future _processExpressionCompilationRequest(request) async {
   final List<String> typeDefinitions = request[6].cast<String>();
   final String libraryUri = request[7];
   final String? klass = request[8];
-  final bool isStatic = request[9];
-  final List<List<int>> dillData = request[10].cast<List<int>>();
-  final int blobLoadCount = request[11];
-  final bool enableAsserts = request[12];
+  final String? method = request[9];
+  final bool isStatic = request[10];
+  final List<List<int>> dillData = request[11].cast<List<int>>();
+  final int blobLoadCount = request[12];
+  final bool enableAsserts = request[13];
   final List<String>? experimentalFlags =
-      request[13] != null ? request[13].cast<String>() : null;
+      request[14] != null ? request[14].cast<String>() : null;
 
   IncrementalCompilerWrapper? compiler = isolateCompilers[isolateGroupId];
 
@@ -618,7 +619,13 @@ Future _processExpressionCompilationRequest(request) async {
   CompilationResult result;
   try {
     Procedure? procedure = await compiler.generator!.compileExpression(
-        expression, definitions, typeDefinitions, libraryUri, klass, isStatic);
+        expression,
+        definitions,
+        typeDefinitions,
+        libraryUri,
+        klass,
+        method,
+        isStatic);
 
     if (procedure == null) {
       port.send(
diff --git a/pkg/vm/lib/incremental_compiler.dart b/pkg/vm/lib/incremental_compiler.dart
index a8d32713a9f..3ad1270525a 100644
--- a/pkg/vm/lib/incremental_compiler.dart
+++ b/pkg/vm/lib/incremental_compiler.dart
@@ -188,11 +188,14 @@ class IncrementalCompiler {
       List<String> typeDefinitions,
       String libraryUri,
       String? klass,
+      String? method,
       bool isStatic) {
     Map<String, DartType> completeDefinitions = {};
-    for (String name in definitions) {
-      if (!isLegalIdentifier(name)) continue;
-      completeDefinitions[name] = new DynamicType();
+    for (int i = 0; i < definitions.length; i++) {
+      String name = definitions[i];
+      if (isLegalIdentifier(name) || (i == 0 && isExtensionThisName(name))) {
+        completeDefinitions[name] = new DynamicType();
+      }
     }
 
     List<TypeParameter> typeParameters = [];
@@ -204,6 +207,7 @@ class IncrementalCompiler {
     Uri library = Uri.parse(libraryUri);
 
     return _generator.compileExpression(expression, completeDefinitions,
-        typeParameters, kDebugProcedureName, library, klass, isStatic);
+        typeParameters, kDebugProcedureName, library,
+        className: klass, methodName: method, isStatic: isStatic);
   }
 }
diff --git a/pkg/vm/test/incremental_compiler_test.dart b/pkg/vm/test/incremental_compiler_test.dart
index 700a5062149..1623621e9bc 100644
--- a/pkg/vm/test/incremental_compiler_test.dart
+++ b/pkg/vm/test/incremental_compiler_test.dart
@@ -112,14 +112,14 @@ main() {
       await compiler.compile();
       compiler.accept();
       {
-        Procedure? procedure = await compiler.compileExpression(
-            'main', <String>[], <String>[], main.uri.toString(), null, true);
+        Procedure? procedure = await compiler.compileExpression('main',
+            <String>[], <String>[], main.uri.toString(), null, null, true);
         expect(procedure, isNotNull);
         expect(errorsReported, equals(0));
       }
       {
-        Procedure? procedure = await compiler.compileExpression(
-            'main1', <String>[], <String>[], main.uri.toString(), null, true);
+        Procedure? procedure = await compiler.compileExpression('main1',
+            <String>[], <String>[], main.uri.toString(), null, null, true);
         expect(procedure, isNotNull);
         expect(errorsReported, equals(1));
         errorsReported = 0;
@@ -1024,8 +1024,8 @@ main() {
       }
       compiler.accept();
       {
-        Procedure? procedure = await compiler.compileExpression(
-            'a', <String>[], <String>[], 'package:foo/bar.dart', 'A', true);
+        Procedure? procedure = await compiler.compileExpression('a', <String>[],
+            <String>[], 'package:foo/bar.dart', 'A', null, true);
         expect(procedure, isNotNull);
       }
 
@@ -1039,8 +1039,8 @@ main() {
       }
       await compiler.reject();
       {
-        Procedure? procedure = await compiler.compileExpression(
-            'a', <String>[], <String>[], 'package:foo/bar.dart', 'A', true);
+        Procedure? procedure = await compiler.compileExpression('a', <String>[],
+            <String>[], 'package:foo/bar.dart', 'A', null, true);
         expect(procedure, isNotNull);
       }
     });
@@ -1088,7 +1088,7 @@ main() {
       compiler.accept();
       {
         final Procedure procedure = (await compiler.compileExpression(
-            'a', <String>[], <String>[], barUri.toString(), 'A', true))!;
+            'a', <String>[], <String>[], barUri.toString(), 'A', null, true))!;
         // Verify that the expression only has links to the only bar we know
         // about.
         final LibraryReferenceCollector lrc = new LibraryReferenceCollector();
@@ -1133,7 +1133,7 @@ main() {
       }
       {
         final Procedure procedure = (await compiler.compileExpression(
-            'a', <String>[], <String>[], barUri.toString(), 'A', true))!;
+            'a', <String>[], <String>[], barUri.toString(), 'A', null, true))!;
         // Verify that the expression only has links to the original bar.
         final LibraryReferenceCollector lrc = new LibraryReferenceCollector();
         procedure.accept(lrc);
diff --git a/runtime/observatory/tests/service/evaluate_in_extension_method_test.dart b/runtime/observatory/tests/service/evaluate_in_extension_method_test.dart
new file mode 100644
index 00000000000..540b1f30aa1
--- /dev/null
+++ b/runtime/observatory/tests/service/evaluate_in_extension_method_test.dart
@@ -0,0 +1,53 @@
+// Copyright (c) 2021, 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:developer';
+import 'package:observatory/models.dart' show InstanceKind;
+import 'package:observatory/service_io.dart';
+import 'package:test/test.dart';
+
+import 'service_test_common.dart';
+import 'test_helper.dart';
+
+extension Foo on String {
+  int parseInt(int x) {
+    debugger();
+    return foo();
+  }
+
+  int foo() => 42;
+}
+
+void testFunction() {
+  print("10".parseInt(21));
+}
+
+var tests = <IsolateTest>[
+  hasStoppedAtBreakpoint,
+  (Isolate isolate) async {
+    Instance result;
+    result = await isolate.evalFrame(0, 'x') as Instance;
+    expect(result.valueAsString, equals('21'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result = await isolate.evalFrame(0, 'this') as Instance;
+    expect(result.valueAsString, equals('10'));
+    expect(result.kind, equals(InstanceKind.string));
+
+    result = await isolate.evalFrame(0, 'foo()') as Instance;
+    expect(result.valueAsString, equals('42'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result = await isolate.evalFrame(0, 'foo() + x') as Instance;
+    expect(result.valueAsString, equals('63'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result =
+        await isolate.evalFrame(0, 'foo() + x + int.parse(this)') as Instance;
+    expect(result.valueAsString, equals('73'));
+    expect(result.kind, equals(InstanceKind.int));
+  },
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);
diff --git a/runtime/observatory/tests/service/service_kernel.status b/runtime/observatory/tests/service/service_kernel.status
index 303c0b690a7..fae5fd02c87 100644
--- a/runtime/observatory/tests/service/service_kernel.status
+++ b/runtime/observatory/tests/service/service_kernel.status
@@ -95,6 +95,7 @@ evaluate_class_type_parameters_test: SkipByDesign # Debugger is disabled in AOT
 evaluate_function_type_parameters_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_async_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_async_star_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
+evaluate_in_extension_method_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_frame_rpc_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_frame_with_scope_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_sync_star_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
diff --git a/runtime/observatory_2/pubspec.yaml b/runtime/observatory_2/pubspec.yaml
index efd6952d6ef..253ce60963f 100644
--- a/runtime/observatory_2/pubspec.yaml
+++ b/runtime/observatory_2/pubspec.yaml
@@ -1,6 +1,6 @@
 name: observatory
 environment:
-  sdk: '>=2.2.2 <3.0.0'
+  sdk: '>=2.6.0 <3.0.0'
 
 dependencies:
   usage: 'any'
diff --git a/runtime/observatory_2/tests/service_2/evaluate_in_extension_method_test.dart b/runtime/observatory_2/tests/service_2/evaluate_in_extension_method_test.dart
new file mode 100644
index 00000000000..5a457481f61
--- /dev/null
+++ b/runtime/observatory_2/tests/service_2/evaluate_in_extension_method_test.dart
@@ -0,0 +1,53 @@
+// Copyright (c) 2021, 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:developer';
+import 'package:observatory_2/models.dart' show InstanceKind;
+import 'package:observatory_2/service_io.dart';
+import 'package:test/test.dart';
+
+import 'service_test_common.dart';
+import 'test_helper.dart';
+
+extension Foo on String {
+  int parseInt(int x) {
+    debugger();
+    return foo();
+  }
+
+  int foo() => 42;
+}
+
+void testFunction() {
+  print("10".parseInt(21));
+}
+
+var tests = <IsolateTest>[
+  hasStoppedAtBreakpoint,
+  (Isolate isolate) async {
+    Instance result;
+    result = await isolate.evalFrame(0, 'x') as Instance;
+    expect(result.valueAsString, equals('21'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result = await isolate.evalFrame(0, 'this') as Instance;
+    expect(result.valueAsString, equals('10'));
+    expect(result.kind, equals(InstanceKind.string));
+
+    result = await isolate.evalFrame(0, 'foo()') as Instance;
+    expect(result.valueAsString, equals('42'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result = await isolate.evalFrame(0, 'foo() + x') as Instance;
+    expect(result.valueAsString, equals('63'));
+    expect(result.kind, equals(InstanceKind.int));
+
+    result =
+        await isolate.evalFrame(0, 'foo() + x + int.parse(this)') as Instance;
+    expect(result.valueAsString, equals('73'));
+    expect(result.kind, equals(InstanceKind.int));
+  },
+];
+
+main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);
diff --git a/runtime/observatory_2/tests/service_2/service_2_kernel.status b/runtime/observatory_2/tests/service_2/service_2_kernel.status
index 16a8edc6f1f..98dd984c2bd 100644
--- a/runtime/observatory_2/tests/service_2/service_2_kernel.status
+++ b/runtime/observatory_2/tests/service_2/service_2_kernel.status
@@ -95,6 +95,7 @@ evaluate_class_type_parameters_test: SkipByDesign # Debugger is disabled in AOT
 evaluate_function_type_parameters_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_async_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_async_star_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
+evaluate_in_extension_method_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_frame_rpc_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_frame_with_scope_test: SkipByDesign # Debugger is disabled in AOT mode.
 evaluate_in_sync_star_activation_test: SkipByDesign # Debugger is disabled in AOT mode.
diff --git a/runtime/vm/compiler_test.cc b/runtime/vm/compiler_test.cc
index 49df77ff3fb..649bca40fe1 100644
--- a/runtime/vm/compiler_test.cc
+++ b/runtime/vm/compiler_test.cc
@@ -209,6 +209,7 @@ TEST_CASE(EvalExpression) {
             /*platform_kernel=*/nullptr, /*platform_kernel_size=*/0,
             expr_text.ToCString(), Array::empty_array(), Array::empty_array(),
             String::Handle(lib_handle.url()).ToCString(), "A",
+            /* method= */ nullptr,
             /* is_static= */ false);
     EXPECT_EQ(Dart_KernelCompilationStatus_Ok, compilation_result.status);
 
diff --git a/runtime/vm/debugger_api_impl_test.cc b/runtime/vm/debugger_api_impl_test.cc
index 51ba8bd596a..f7b5cfde113 100644
--- a/runtime/vm/debugger_api_impl_test.cc
+++ b/runtime/vm/debugger_api_impl_test.cc
@@ -184,6 +184,7 @@ DART_EXPORT Dart_Handle Dart_EvaluateStaticExpr(Dart_Handle lib_handle,
             /* type_defintions= */ Array::empty_array(),
             String::Handle(lib.url()).ToCString(),
             /* klass= */ nullptr,
+            /* method= */ nullptr,
             /* is_static= */ true);
     if (compilation_result.status != Dart_KernelCompilationStatus_Ok) {
       return Api::NewError("Failed to compile expression.");
diff --git a/runtime/vm/kernel_isolate.cc b/runtime/vm/kernel_isolate.cc
index 4156252e4b3..c1df5921ee5 100644
--- a/runtime/vm/kernel_isolate.cc
+++ b/runtime/vm/kernel_isolate.cc
@@ -480,6 +480,7 @@ class KernelCompilationRequest : public ValueObject {
       const Array& type_definitions,
       char const* library_uri,
       char const* klass,
+      char const* method,
       bool is_static,
       const MallocGrowableArray<char*>* experimental_flags) {
     if (port_ == ILLEGAL_PORT) {
@@ -565,6 +566,14 @@ class KernelCompilationRequest : public ValueObject {
       class_object.type = Dart_CObject_kNull;
     }
 
+    Dart_CObject method_object;
+    if (method != NULL) {
+      method_object.type = Dart_CObject_kString;
+      method_object.value.as_string = const_cast<char*>(method);
+    } else {
+      method_object.type = Dart_CObject_kNull;
+    }
+
     Dart_CObject is_static_object;
     is_static_object.type = Dart_CObject_kBool;
     is_static_object.value.as_bool = is_static;
@@ -654,6 +663,7 @@ class KernelCompilationRequest : public ValueObject {
                                    &type_definitions_object,
                                    &library_uri_object,
                                    &class_object,
+                                   &method_object,
                                    &is_static_object,
                                    &dills_object,
                                    &num_blob_loads,
@@ -1133,6 +1143,7 @@ Dart_KernelCompilationResult KernelIsolate::CompileExpressionToKernel(
     const Array& type_definitions,
     const char* library_url,
     const char* klass,
+    const char* method,
     bool is_static) {
   Dart_Port kernel_port = WaitForKernelPort();
   if (kernel_port == ILLEGAL_PORT) {
@@ -1147,7 +1158,7 @@ Dart_KernelCompilationResult KernelIsolate::CompileExpressionToKernel(
   ASSERT(is_static || (klass != nullptr));
   return request.SendAndWaitForResponse(
       kernel_port, platform_kernel, platform_kernel_size, expression,
-      definitions, type_definitions, library_url, klass, is_static,
+      definitions, type_definitions, library_url, klass, method, is_static,
       experimental_flags_);
 }
 
diff --git a/runtime/vm/kernel_isolate.h b/runtime/vm/kernel_isolate.h
index 2ba4f2c2df4..5880beb0e71 100644
--- a/runtime/vm/kernel_isolate.h
+++ b/runtime/vm/kernel_isolate.h
@@ -75,6 +75,7 @@ class KernelIsolate : public AllStatic {
       const Array& type_definitions,
       const char* library_url,
       const char* klass,
+      const char* method,
       bool is_static);
 
   static Dart_KernelCompilationResult ListDependencies();
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 17b42b696f2..1b05ac5c774 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -2721,6 +2721,7 @@ static void BuildExpressionEvaluationScope(Thread* thread, JSONStream* js) {
   const GrowableObjectArray& type_params_names =
       GrowableObjectArray::Handle(zone, GrowableObjectArray::New());
   String& klass_name = String::Handle(zone);
+  String& method_name = String::Handle(zone);
   String& library_uri = String::Handle(zone);
   bool isStatic = false;
 
@@ -2746,11 +2747,13 @@ static void BuildExpressionEvaluationScope(Thread* thread, JSONStream* js) {
         klass_name = cls.UserVisibleName();
       }
       library_uri = Library::Handle(zone, cls.library()).url();
+      method_name = frame->function().UserVisibleName();
       isStatic = true;
     } else {
       const Class& method_cls = Class::Handle(zone, frame->function().origin());
       library_uri = Library::Handle(zone, method_cls.library()).url();
       klass_name = method_cls.UserVisibleName();
+      method_name = frame->function().UserVisibleName();
       isStatic = false;
     }
   } else {
@@ -2829,6 +2832,9 @@ static void BuildExpressionEvaluationScope(Thread* thread, JSONStream* js) {
   if (!klass_name.IsNull()) {
     report.AddProperty("klass", klass_name.ToCString());
   }
+  if (!method_name.IsNull()) {
+    report.AddProperty("method", method_name.ToCString());
+  }
   report.AddProperty("isStatic", isStatic);
 }
 
@@ -2877,6 +2883,7 @@ static const MethodParameter* const compile_expression_params[] = {
     new StringParameter("libraryUri", true),
     new StringParameter("klass", false),
     new BoolParameter("isStatic", false),
+    new StringParameter("method", false),
     NULL,
 };
 
@@ -2922,7 +2929,8 @@ static void CompileExpression(Thread* thread, JSONStream* js) {
           kernel_buffer, kernel_buffer_len, js->LookupParam("expression"),
           Array::Handle(Array::MakeFixedLength(params)),
           Array::Handle(Array::MakeFixedLength(type_params)),
-          js->LookupParam("libraryUri"), js->LookupParam("klass"), is_static);
+          js->LookupParam("libraryUri"), js->LookupParam("klass"),
+          js->LookupParam("method"), is_static);
 
   if (compilation_result.status != Dart_KernelCompilationStatus_Ok) {
     js->PrintError(kExpressionCompilationError, "%s", compilation_result.error);
diff --git a/runtime/vm/unit_test.cc b/runtime/vm/unit_test.cc
index cb67c6fc665..e96548eb77d 100644
--- a/runtime/vm/unit_test.cc
+++ b/runtime/vm/unit_test.cc
@@ -648,7 +648,8 @@ Dart_Handle TestCase::EvaluateExpression(const Library& lib,
         KernelIsolate::CompileExpressionToKernel(
             /* platform_kernel= */ nullptr, /* platform_kernel_size= */ 0,
             expr.ToCString(), param_names, Array::empty_array(),
-            String::Handle(lib.url()).ToCString(), /* klass=*/nullptr,
+            String::Handle(lib.url()).ToCString(), /* klass= */ nullptr,
+            /* method= */ nullptr,
             /* is_static= */ true);
     if (compilation_result.status != Dart_KernelCompilationStatus_Ok) {
       return Api::NewError("%s", compilation_result.error);
diff --git a/sdk/lib/vmservice/running_isolates.dart b/sdk/lib/vmservice/running_isolates.dart
index f9db4a5058d..17ba44877b1 100644
--- a/sdk/lib/vmservice/running_isolates.dart
+++ b/sdk/lib/vmservice/running_isolates.dart
@@ -132,6 +132,10 @@ class _Evaluator {
     if (klass != null) {
       compileParams['klass'] = klass;
     }
+    final method = buildScopeResponseResult['method'];
+    if (method != null) {
+      compileParams['method'] = method;
+    }
     if (externalClient != null) {
       final compileExpression = Message.forMethod('compileExpression');
       compileExpression.client = externalClient;