dart2wasm: Prepare standalone target

When compiled to WebAssembly, the Dart SDK needs access to external
host functions to implement regular expressions, stack traces, timers
and more. Currently, `dart2wasm` relies on `js_interop` definition to
implement these functions in JavaScript.

As discussed in https://github.com/dart-lang/sdk/issues/53884, an
alternative is to use `wasm:import` annotations to let an arbitrary
embedder that doesn't necessarily run in a JavaScript context inject
implementations for these host functions.

This would allow running `dart2wasm` apps by e.g.

  - using a runtime like wasmtime and defining host functions in Rust.
  - defining a wrapper module implementing required functions by
    delegating to WASI definitions, and then using say `wasm-merge` to
    run the app in any WASI-compatible runtime.

This prepares the `--standalone` flag on `dart2wasm` to do just that.
When enabled, the compiler uses a different SDK platform to use imports
instead of JS interop. For now, these platforms are almost identical:
I've ported the timer logic to use wasm imports as a demo, but the rest
is still based on existing patch files. We can revisit in subsequent
CLs to incrementally reduce `js_interop` dependencies before removing
that library from the `dart2wasm_standalone` target entirely.

Change-Id: I3f406afbf2dab65506094de5c3f4067f4db66f3e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/486380
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

BUILD.gn

@@ -162,6 +162,7 @@ group("dart2wasm_platform") {
     ":runtime_precompiled",
     "utils/dart2wasm:compile_dart2wasm_js_compatibility_platform",
     "utils/dart2wasm:compile_dart2wasm_platform",
+    "utils/dart2wasm:compile_dart2wasm_standalone_platform",
     "utils/dart2wasm:dart2wasm_snapshot",
   ]
   if (defined(is_product)) {

pkg/dart2wasm/lib/compile.dart

@@ -215,6 +215,8 @@ Future<CompilationResult> compile(
   final wasm.Mode mode;
   if (options.translatorOptions.jsCompatibility) {
     mode = wasm.Mode.jsCompatibility;
+  } else if (options.translatorOptions.standalone) {
+    mode = wasm.Mode.standalone;
   } else {
     mode = wasm.Mode.regular;
   }

pkg/dart2wasm/lib/dart2wasm.dart

@@ -43,6 +43,10 @@ final List<Option> options = [
     o.translatorOptions.jsCompatibility = value;
     o.environment['dart.wasm.js_compatibility'] = 'true';
   }, defaultsTo: _d.translatorOptions.jsCompatibility),
+  Flag("standalone", (o, value) {
+    o.translatorOptions.standalone = value;
+    o.environment['dart.wasm.standalone'] = 'true';
+  }, defaultsTo: _d.translatorOptions.standalone),
   Flag(
       "enable-asserts", (o, value) => o.translatorOptions.enableAsserts = value,
       defaultsTo: _d.translatorOptions.enableAsserts),

pkg/dart2wasm/lib/target.dart

@@ -42,6 +42,7 @@ import 'wasm_library_checks.dart' as wasmChecks;
 enum Mode {
   regular,
   jsCompatibility,
+  standalone,
 }
 
 class Dart2WasmConstantsBackend extends ConstantsBackend {
@@ -129,14 +130,16 @@ class WasmTarget extends Target {
   String get name {
     return switch (mode) {
       Mode.regular => 'wasm',
-      Mode.jsCompatibility => 'wasm_js_compatibility'
+      Mode.jsCompatibility => 'wasm_js_compatibility',
+      Mode.standalone => 'wasm_standalone',
     };
   }
 
   String get platformFile {
     return switch (mode) {
       Mode.regular => 'dart2wasm_platform.dill',
-      Mode.jsCompatibility => 'dart2wasm_js_compatibility_platform.dill'
+      Mode.jsCompatibility => 'dart2wasm_js_compatibility_platform.dill',
+      Mode.standalone => 'dart2wasm_standalone_platform.dill',
     };
   }
 

pkg/dart2wasm/lib/translator.dart

@@ -50,6 +50,7 @@ class TranslatorOptions {
   int optimizationLevel = 1;
   bool? inliningOverride;
   bool jsCompatibility = false;
+  bool standalone = false;
   bool? omitImplicitTypeChecksOverride;
   bool omitExplicitTypeChecks = false;
   bool? omitBoundsChecksOverride;
@@ -84,6 +85,7 @@ class TranslatorOptions {
     sink.writeInt(optimizationLevel);
     sink.writeNullable(inliningOverride, sink.writeBool);
     sink.writeBool(jsCompatibility);
+    sink.writeBool(standalone);
     sink.writeNullable(omitImplicitTypeChecksOverride, sink.writeBool);
     sink.writeBool(omitExplicitTypeChecks);
     sink.writeNullable(omitBoundsChecksOverride, sink.writeBool);
@@ -112,6 +114,7 @@ class TranslatorOptions {
     options.optimizationLevel = source.readInt();
     options.inliningOverride = source.readNullable(source.readBool);
     options.jsCompatibility = source.readBool();
+    options.standalone = source.readBool();
     options.omitImplicitTypeChecksOverride =
         source.readNullable(source.readBool);
     options.omitExplicitTypeChecks = source.readBool();

pkg/dart2wasm/tool/compile_benchmark

@@ -90,6 +90,12 @@ while [ $# -gt 0 ]; do
       shift
       ;;
 
+    --standalone)
+      PLATFORM_FILENAME="$BIN_DIR/dart2wasm_standalone_platform.dill"
+      DART2WASM_ARGS+=("--standalone")
+      shift
+      ;;
+
     --compiler-asserts)
       SNAPSHOT_NAME="dart2wasm_asserts"
       VM_ARGS+=("--enable-asserts")

pkg/front_end/lib/src/codes/diagnostic.g.dart

@@ -3467,7 +3467,7 @@ const MessageCode fastaUsageLong = const MessageCode(
     Read the SDK platform from <file>, which should be in Dill/Kernel IR format
     and contain the Dart SDK.
 
-  --target=dart2js|dart2js_server|dart2wasm|dart2wasm_js_compatibility|dart_runner|dartdevc|flutter|flutter_runner|none|vm
+  --target=dart2js|dart2js_server|dart2wasm|dart2wasm_js_compatibility|dart2wasm_standalone|dart_runner|dartdevc|flutter|flutter_runner|none|vm
     Specify the target configuration.
 
   --enable-asserts

pkg/front_end/messages.yaml

@@ -2013,7 +2013,7 @@ fastaUsageLong:
         Read the SDK platform from <file>, which should be in Dill/Kernel IR format
         and contain the Dart SDK.
 
-      --target=dart2js|dart2js_server|dart2wasm|dart2wasm_js_compatibility|dart_runner|dartdevc|flutter|flutter_runner|none|vm
+      --target=dart2js|dart2js_server|dart2wasm|dart2wasm_js_compatibility|dart2wasm_standalone|dart_runner|dartdevc|flutter|flutter_runner|none|vm
         Specify the target configuration.
 
       --enable-asserts

pkg/front_end/test/spell_checking_list_common.txt

@@ -762,6 +762,7 @@ dart
 dart2js
 dart2wasm
 dart2wasm_js_compatibility
+dart2wasm_standalone
 dartdevc
 data
 date

pkg/front_end/tool/additional_targets.dart

@@ -22,5 +22,7 @@ void installAdditionalTargets() {
   targets["dart2wasm"] = (TargetFlags flags) => new WasmTarget();
   targets["dart2wasm_js_compatibility"] = (TargetFlags flags) =>
       new WasmTarget(mode: wasm.Mode.jsCompatibility);
+  targets["dart2wasm_standalone"] = (TargetFlags flags) =>
+      new WasmTarget(mode: wasm.Mode.standalone);
   vm_target_install.installAdditionalTargets();
 }

pkg/test_runner/lib/src/browser.dart

@@ -321,8 +321,30 @@ $script
 """;
 }
 
-String dart2wasmHtml(
-    String title, String wasmPath, String mjsPath, String supportJsPath) {
+String dart2wasmHtml(String title, String wasmPath, String mjsPath,
+    String supportJsPath, bool standalone) {
+  const standaloneEmbedder = """
+    const dartEmbedder = {
+      // See sdk/lib/_internal/wasm_standalone/lib/embedder.dart for required definitions.
+      scheduleOnce: (delayInMicros, callback, arg) => {
+        const timeout = setTimeout(() => callback(arg), Number(delayInMicros / 1000n));
+        return {timeout};
+      },
+      scheduleRepeated: (intervalMicros, callback, arg) => {
+        const timeout = setInterval(() => callback(arg), Number(intervalMicros / 1000));
+        return {timeout};
+      },
+      queueMicrotask: (callback, arg) => {
+        queueMicrotask(() => callback(arg));
+      },
+      clearSchedule: (schedule) => {
+        clearTimeout(schedule.timeout);
+      },
+      currentTime: () => BigInt(Date.now()) * 1000n,
+    };
+""";
+  final additionalImports = standalone ? '{ dart: dartEmbedder }' : '{}';
+
   return """
 <!DOCTYPE html>
 <html>
@@ -365,7 +387,8 @@ String dart2wasmHtml(
       const response = await fetch(`\${path}/\${relativeToWasmFileUri}`);
       return response.arrayBuffer();
     };
-    const appInstance = await compiledApp.instantiate({}, {
+${standalone ? standaloneEmbedder : ''}
+    const appInstance = await compiledApp.instantiate($additionalImports, {
       loadDeferredModules: (modules, handleWasmBytes) =>
         Promise.all(modules.map((m) => fetch(m).then((b) => handleWasmBytes(m, b)))),
     });

pkg/test_runner/lib/src/test_suite.dart

@@ -947,8 +947,11 @@ class StandardTestSuite extends TestSuite {
             _createUrlPathFromFile(Path('$outputDir/$nameNoExt.mjs'));
         final supportJsPath =
             _createUrlPathFromFile(Path('$outputDir/$nameNoExt.support.js'));
-        content = dart2wasmHtml(
-            testFile.path.toNativePath(), wasmPath, mjsPath, supportJsPath);
+
+        final isStandalone = configuration.dart2wasmOptions
+            .any((e) => e.contains('--standalone'));
+        content = dart2wasmHtml(testFile.path.toNativePath(), wasmPath, mjsPath,
+            supportJsPath, isStandalone);
       } else if (configuration.compiler == Compiler.ddc) {
         var ddcConfig =
             configuration.compilerConfiguration as DevCompilerConfiguration;

sdk/lib/_internal/wasm/lib/async_patch.dart

@@ -1,11 +1,7 @@
-import 'dart:_internal' show _AsyncCompleter, patch, exportWasmFunction;
-
-import 'dart:_js_helper' show JS;
+import 'dart:_internal' show _AsyncCompleter, patch;
 
 import 'dart:_wasm';
 
-part 'timer_patch.dart';
-
 // Modular kernel transformer will make calls to this method be re-directed to
 // call dart:core:Error._trySetStackTrace instead.
 @patch

sdk/lib/_internal/wasm/lib/timer_patch.dart

@@ -2,7 +2,9 @@
 // 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.
 
-part of "async_patch.dart";
+import 'dart:_internal' show patch, exportWasmFunction;
+
+import 'dart:_js_helper' show JS;
 
 // Implementation of `Timer` and `scheduleMicrotask` via the JS event loop.
 

sdk/lib/_internal/wasm_standalone/OWNERS

@@ -0,0 +1 @@
+file:/tools/OWNERS_WASM

sdk/lib/_internal/wasm_standalone/lib/embedder.dart

@@ -0,0 +1,57 @@
+// Copyright (c) 2026, 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.
+
+/// Imported definitions that need to be provided to run `dart2wasm` apps
+/// without JavaScript.
+///
+/// These definitions are currently incomplete. Once all externs in the Dart SDK
+/// are implemented with these imports, the `dart2wasm_standalone` target can
+/// run without `js_interop`. This will enable embedders without JavaScript
+/// support to run `dart2wasm` apps by either:
+///
+///  - providing `dartrt` imports when instantiating the module.
+///  - using an external tool like `wasm-merge` to link another module that
+///    could provide implementations by e.g. delegating to WASI definitions.
+library;
+
+import 'dart:core';
+import 'dart:core' as core;
+import 'dart:_wasm';
+
+/// Instructs the runtime to invoke `callback(arg)` after the delay in
+/// microseconds.
+///
+/// Returns a handle that can be used with [clearSchedule] to abort the timer.
+@pragma("wasm:import", "dart.scheduleOnce")
+external WasmExternRef scheduleOnce(
+  WasmI64 delay,
+  WasmFunction<void Function(WasmAnyRef)> callback,
+  WasmAnyRef arg,
+);
+
+/// Instructs the runtime to invoke `callback(arg)` every `interval`
+/// microseconds.
+///
+/// Returns a handle that can be used with [clearSchedule] to abort the timer.
+@pragma("wasm:import", "dart.scheduleRepeated")
+external WasmExternRef scheduleRepeated(
+  WasmI64 interval,
+  WasmFunction<void Function(WasmAnyRef)> callback,
+  WasmAnyRef arg,
+);
+
+/// Instructs the runtime to invoke `callback(arg)` before returning to the
+/// event loop.
+@pragma("wasm:import", "dart.queueMicrotask")
+external void queueMicrotask(
+  WasmFunction<void Function(WasmAnyRef)> callback,
+  WasmAnyRef arg,
+);
+
+/// Cancels a schedule created through [scheduleOnce] or [scheduleRepeated].
+@pragma("wasm:import", "dart.clearSchedule")
+external void clearSchedule(WasmExternRef? schedule);
+
+@pragma("wasm:import", "dart.currentTime")
+external WasmI64 currentTimeMicros();

sdk/lib/_internal/wasm_standalone/lib/timer_patch.dart

@@ -0,0 +1,124 @@
+// Copyright (c) 2026, 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:_internal' show patch;
+import 'dart:_embedder' as embedder;
+import 'dart:_wasm';
+
+@patch
+class Timer {
+  @patch
+  static Timer _createTimer(Duration duration, void callback()) {
+    return _OneShotTimer(duration, callback);
+  }
+
+  @patch
+  static Timer _createPeriodicTimer(
+    Duration duration,
+    void callback(Timer timer),
+  ) {
+    return _PeriodicTimer(duration, callback);
+  }
+}
+
+abstract class _Timer implements Timer {
+  final int _microseconds;
+  int _tick;
+  WasmExternRef? _handle;
+
+  @override
+  int get tick => _tick;
+
+  @override
+  bool get isActive => _handle != null;
+
+  _Timer(Duration duration)
+    : _microseconds = duration.inMicroseconds,
+      _tick = 0,
+      _handle = null {
+    _schedule();
+  }
+
+  void _schedule();
+  void _processTick();
+
+  @override
+  void cancel() {
+    if (!_handle.isNull) {
+      embedder.clearSchedule(_handle);
+      _handle = WasmExternRef.nullRef;
+    }
+  }
+
+  static void runtimeCallback(WasmAnyRef timer) {
+    final dartTimer = timer.toObject() as _Timer;
+    dartTimer._processTick();
+  }
+}
+
+class _OneShotTimer extends _Timer {
+  final void Function() _callback;
+
+  _OneShotTimer(Duration duration, this._callback) : super(duration);
+
+  @override
+  void _schedule() {
+    _handle = embedder.scheduleOnce(
+      _microseconds.toWasmI64(),
+      WasmFunction.fromFunction(_Timer.runtimeCallback),
+      WasmAnyRef.fromObject(this),
+    );
+  }
+
+  _processTick() {
+    _tick++;
+    _handle = null;
+    _callback();
+  }
+}
+
+class _PeriodicTimer extends _Timer {
+  final void Function(Timer) _callback;
+  int _start = 0;
+
+  _PeriodicTimer(Duration duration, this._callback) : super(duration);
+
+  @override
+  void _schedule() {
+    _start = embedder.currentTimeMicros().toInt();
+    _handle = embedder.scheduleRepeated(
+      _microseconds.toWasmI64(),
+      WasmFunction.fromFunction(_Timer.runtimeCallback),
+      WasmAnyRef.fromObject(this),
+    );
+  }
+
+  @override
+  void _processTick() {
+    _tick++;
+    if (_microseconds > 0) {
+      final int duration = embedder.currentTimeMicros().toInt() - _start;
+      if (duration > _tick * _microseconds) {
+        _tick = duration ~/ _microseconds;
+      }
+    }
+    _callback(this);
+  }
+}
+
+@patch
+class _AsyncRun {
+  @patch
+  static void _scheduleImmediate(void callback()) {
+    embedder.queueMicrotask(
+      WasmFunction.fromFunction(_runtimeCallback),
+      WasmAnyRef.fromObject(callback),
+    );
+  }
+
+  static void _runtimeCallback(WasmAnyRef callbackFunction) {
+    final function = callbackFunction.toObject() as void Function();
+    function();
+  }
+}

sdk/lib/libraries.json

@@ -134,7 +134,7 @@
   "wasm": {
     "include": [
       {
-        "target": "wasm_common"
+        "target": "wasm_js_common"
       }
     ],
     "libraries": {
@@ -192,7 +192,7 @@
   "wasm_js_compatibility": {
     "include": [
       {
-        "target": "wasm_common"
+        "target": "wasm_js_common"
       }
     ],
     "libraries": {
@@ -244,6 +244,90 @@
       }
     }
   },
+  "wasm_standalone": {
+    "include": [
+      {
+        "target": "wasm_common"
+      }
+    ],
+    "libraries": {
+      "_embedder": {
+        "uri": "_internal/wasm_standalone/lib/embedder.dart"
+      },
+      "async": {
+        "uri": "async/async.dart",
+        "patches": [
+          "_internal/wasm/lib/async_patch.dart",
+          "_internal/wasm_standalone/lib/timer_patch.dart"
+        ]
+      },
+      "core": {
+        "uri": "core/core.dart",
+        "patches": [
+          "_internal/vm_shared/lib/bigint_patch.dart",
+          "_internal/vm_shared/lib/bool_patch.dart",
+          "_internal/vm_shared/lib/date_patch.dart",
+          "_internal/vm_shared/lib/map_patch.dart",
+          "_internal/vm_shared/lib/null_patch.dart",
+          "_internal/wasm/lib/array_patch.dart",
+          "_internal/wasm/lib/bigint_patch_patch.dart",
+          "_internal/wasm/lib/core_patch.dart",
+          "_internal/wasm/lib/date_patch_patch.dart",
+          "_internal/wasm/lib/int_common_patch.dart",
+          "_internal/wasm/lib/int_patch.dart",
+          "_internal/wasm/lib/string_buffer_patch.dart",
+          "_internal/wasm/lib/string_patch.dart",
+          "_internal/wasm/lib/sync_star_patch.dart",
+          "_internal/wasm/lib/weak_patch.dart"
+        ]
+      },
+      "convert": {
+        "uri": "convert/convert.dart",
+        "patches": [
+          "_internal/wasm/lib/convert_patch.dart"
+        ]
+      },
+      "typed_data": {
+        "uri": "typed_data/typed_data.dart",
+        "patches": [
+          "_internal/wasm/lib/simd_patch.dart",
+          "_internal/wasm/lib/typed_data_patch.dart"
+        ]
+      },
+      "_boxed_int": {
+        "uri": "_internal/wasm/lib/boxed_int.dart",
+        "patches": "_internal/wasm/lib/boxed_int_to_string.dart"
+      },
+      "_string": {
+        "uri": "_internal/wasm/lib/js_string.dart"
+      },
+      "_typed_data": {
+        "uri": "_internal/wasm/lib/typed_data.dart"
+      },
+      "_js_helper": {
+        "uri": "_internal/wasm/lib/js_helper.dart",
+        "patches": [
+          "_internal/wasm/lib/js_helper_patch.dart"
+        ]
+      }
+    }
+  },
+  "wasm_js_common": {
+    "include": [
+      {
+        "target": "wasm_common"
+      }
+    ],
+    "libraries": {
+      "async": {
+        "uri": "async/async.dart",
+        "patches": [
+          "_internal/wasm/lib/async_patch.dart",
+          "_internal/wasm/lib/timer_patch.dart"
+        ]
+      }
+    }
+  },
   "wasm_common": {
     "libraries": {
       "core": {
@@ -301,12 +385,6 @@
         "uri": "_wasm/wasm_types.dart",
         "patches": "_internal/wasm/lib/wasm_types_patch.dart"
       },
-      "async": {
-        "uri": "async/async.dart",
-        "patches": [
-          "_internal/wasm/lib/async_patch.dart"
-        ]
-      },
       "collection": {
         "uri": "collection/collection.dart",
         "patches": [

sdk/lib/libraries.yaml

@@ -125,7 +125,7 @@ vm:
 
 wasm:
   include:
-    - target: "wasm_common"
+    - target: "wasm_js_common"
   libraries:
     core:
       uri: core/core.dart
@@ -169,7 +169,7 @@ wasm:
 
 wasm_js_compatibility:
   include:
-    - target: "wasm_common"
+    - target: "wasm_js_common"
   libraries:
     convert:
       uri: convert/convert.dart
@@ -209,6 +209,69 @@ wasm_js_compatibility:
       patches:
         - _internal/wasm_js_compatibility/lib/js_helper_patch.dart
 
+wasm_standalone:
+  include:
+    - target: "wasm_common"
+  libraries:
+    _embedder:
+      uri: _internal/wasm_standalone/lib/embedder.dart
+    async:
+      uri: async/async.dart
+      patches:
+      - _internal/wasm/lib/async_patch.dart
+      - _internal/wasm_standalone/lib/timer_patch.dart
+
+    # Not yet migrated
+    core:
+      uri: core/core.dart
+      patches:
+        - _internal/vm_shared/lib/bigint_patch.dart
+        - _internal/vm_shared/lib/bool_patch.dart
+        - _internal/vm_shared/lib/date_patch.dart
+        - _internal/vm_shared/lib/map_patch.dart
+        - _internal/vm_shared/lib/null_patch.dart
+        - _internal/wasm/lib/array_patch.dart
+        - _internal/wasm/lib/bigint_patch_patch.dart
+        - _internal/wasm/lib/core_patch.dart
+        - _internal/wasm/lib/date_patch_patch.dart
+        - _internal/wasm/lib/int_common_patch.dart
+        - _internal/wasm/lib/int_patch.dart
+        - _internal/wasm/lib/string_buffer_patch.dart
+        - _internal/wasm/lib/string_patch.dart
+        - _internal/wasm/lib/sync_star_patch.dart
+        - _internal/wasm/lib/weak_patch.dart
+    convert:
+      uri: convert/convert.dart
+      patches:
+        - _internal/wasm/lib/convert_patch.dart
+    typed_data:
+      uri: typed_data/typed_data.dart
+      patches:
+        - _internal/wasm/lib/simd_patch.dart
+        - _internal/wasm/lib/typed_data_patch.dart
+    _boxed_int:
+      uri: _internal/wasm/lib/boxed_int.dart
+      patches:
+        _internal/wasm/lib/boxed_int_to_string.dart
+    _string:
+      uri: _internal/wasm/lib/js_string.dart
+    _typed_data:
+      uri: _internal/wasm/lib/typed_data.dart
+    _js_helper:
+      uri: _internal/wasm/lib/js_helper.dart
+      patches:
+        - _internal/wasm/lib/js_helper_patch.dart
+
+wasm_js_common:
+  include:
+    - target: "wasm_common"
+  libraries:
+    async:
+      uri: async/async.dart
+      patches:
+      - _internal/wasm/lib/async_patch.dart
+      - _internal/wasm/lib/timer_patch.dart
+
 wasm_common:
   libraries:
     core:
@@ -248,10 +311,6 @@ wasm_common:
     _wasm:
       uri: _wasm/wasm_types.dart
       patches: _internal/wasm/lib/wasm_types_patch.dart
-    async:
-      uri: async/async.dart
-      patches:
-      - _internal/wasm/lib/async_patch.dart
     collection:
       uri: collection/collection.dart
       patches:

tools/bots/test_matrix.json

@@ -54,6 +54,8 @@
       "out/ReleaseX64/dart2wasm_platform.dill",
       "out/ReleaseX64/dart2wasm_js_compatibility_outline.dill",
       "out/ReleaseX64/dart2wasm_js_compatibility_platform.dill",
+      "out/ReleaseX64/dart2wasm_standalone_outline.dill",
+      "out/ReleaseX64/dart2wasm_standalone_platform.dill",
       "out/ReleaseX64/dart-sdk/",
       "out/ReleaseX64/wasm/",
       "out/ReleaseX64/wasm-opt",
@@ -527,6 +529,17 @@
         "timeout": 60
       }
     },
+    "dart2wasm-(linux|mac|win)-standalone-(chrome|firefox|safari)": {
+      "options": {
+        "dart2wasm-options": [
+          "-O0",
+          "--standalone"
+        ],
+        "host-asserts": true,
+        "use-sdk": false,
+        "timeout": 60
+      }
+    },
     "vm-aot-android-(debug|product|release)-arm_x64": {
       "options": {
         "builder-tag": "crossword",
@@ -2889,6 +2902,37 @@
         }
       ]
     },
+    {
+      "builders": [
+        "dart2wasm-linux-standalone-chrome"
+      ],
+      "meta": {
+        "description": "dart2wasm standalone tests"
+      },
+      "steps": [
+        {
+          "name": "build dart",
+          "script": "tools/build.py",
+          "arguments": [
+            "runtime",
+            "dart2wasm",
+            "dartaotruntime"
+          ]
+        },
+        {
+          "name": "dart2wasm tests",
+          "arguments": [
+            "-ndart2wasm-${system}-standalone-${runtime}",
+            "co19",
+            "corelib",
+            "language",
+            "lib"
+          ],
+          "shards": 8,
+          "fileset": "dart2wasm_hostasserts"
+        }
+      ]
+    },
     {
       "builders": [
         "analyzer-linux-release",

utils/dart2wasm/BUILD.gn

@@ -75,6 +75,22 @@ compile_platform("compile_dart2wasm_js_compatibility_platform") {
   ]
 }
 
+compile_platform("compile_dart2wasm_standalone_platform") {
+  single_root_scheme = "org-dartlang-sdk"
+  single_root_base = rebase_path("$sdk_root/")
+  libraries_specification_uri = "org-dartlang-sdk:///lib/libraries.json"
+
+  outputs = [
+    "$root_out_dir/dart2wasm_standalone_platform.dill",
+    "$root_out_dir/dart2wasm_standalone_outline.dill",
+  ]
+
+  args = [
+    "--target=dart2wasm_standalone",
+    "dart:core",
+  ]
+}
+
 wasm_module("ffi_native_test_wasm_module") {
   module_name = "ffi_native_test_module"
 }