Implement sass --embedded in pure JS mode

bin/sass.dart

@@ -18,8 +18,7 @@ import 'package:sass/src/io.dart';
 import 'package:sass/src/stylesheet_graph.dart';
 import 'package:sass/src/utils.dart';
 import 'package:sass/src/embedded/executable.dart'
-    // Never load the embedded protocol when compiling to JS.
-    if (dart.library.js) 'package:sass/src/embedded/unavailable.dart'
+    if (dart.library.js) 'package:sass/src/embedded/js/executable.dart'
     as embedded;
 
 Future<void> main(List<String> args) async {

lib/src/embedded/compilation_dispatcher.dart

@@ -4,10 +4,11 @@
 
 import 'dart:convert';
 import 'dart:io';
-import 'dart:isolate';
+import 'dart:isolate' if (dart.library.js) './js/isolate.dart';
 import 'dart:typed_data';
 
-import 'package:native_synchronization/mailbox.dart';
+import 'package:native_synchronization/mailbox.dart'
+    if (dart.library.js) './js/mailbox.dart';
 import 'package:path/path.dart' as p;
 import 'package:protobuf/protobuf.dart';
 import 'package:pub_semver/pub_semver.dart';
@@ -392,3 +393,6 @@ final class CompilationDispatcher {
     }
   }
 }
+
+void spawnCompilationDispatcher(Mailbox mailbox, SendPort sendPort) =>
+    CompilationDispatcher(mailbox, sendPort).listen();

lib/src/embedded/js/executable.dart

@@ -0,0 +1,20 @@
+// Copyright 2024 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'dart:js_interop';
+
+import '../compilation_dispatcher.dart';
+
+@JS('sass_embedded')
+extension type SassEmbedded._(JSObject o) implements JSObject {
+  external static void main(JSExportedDartFunction createCompilationDispatcher);
+}
+
+void main(List<String> args) {
+  try {
+    SassEmbedded.main(spawnCompilationDispatcher.toJS);
+  } catch (error) {
+    print(error);
+  }
+}

lib/src/embedded/js/isolate.dart

@@ -0,0 +1,24 @@
+// Copyright 2024 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'dart:js_interop';
+import 'dart:typed_data';
+
+/// MessagePort from JS wrapped as SendPort in Dart
+@JS()
+extension type SendPort._(JSObject o) implements JSObject {
+  @JS('postMessage')
+  external void _postMessage(JSAny message);
+  void send(Uint8List message) => _postMessage(message.toJS);
+}
+
+@JS()
+extension type Isolate._(JSObject o) implements JSObject {
+  static Never exit([SendPort? finalMessagePort, Uint8List? message]) {
+    if (message != null) {
+      finalMessagePort?.send(message);
+    }
+    throw Error();
+  }
+}

lib/src/embedded/js/mailbox.dart

@@ -0,0 +1,14 @@
+// Copyright 2024 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'dart:js_interop';
+import 'dart:typed_data';
+
+/// SyncMessagePort from JS wrapped as Mailbox in Dart
+@JS()
+extension type Mailbox._(JSObject o) implements JSObject {
+  @JS('receiveMessage')
+  external JSAny _receiveMessage();
+  Uint8List take() => (_receiveMessage() as JSUint8Array).toDart;
+}

lib/src/embedded/utils.dart

@@ -2,14 +2,14 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'dart:io';
 import 'dart:typed_data';
 
 import 'package:protobuf/protobuf.dart';
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:term_glyph/term_glyph.dart' as term_glyph;
 
+import '../io.dart';
 import '../syntax.dart';
 import 'embedded_sass.pb.dart' as proto;
 import 'embedded_sass.pb.dart' hide SourceSpan, Syntax;
@@ -136,15 +136,17 @@ ProtocolError handleError(Object error, StackTrace stackTrace,
     {int? messageId}) {
   if (error is ProtocolError) {
     error.id = messageId ?? errorId;
-    stderr.write("Host caused ${error.type.name.toLowerCase()} error");
-    if (error.id != errorId) stderr.write(" with request ${error.id}");
-    stderr.writeln(": ${error.message}");
+    var buffer = StringBuffer();
+    buffer.write("Host caused ${error.type.name.toLowerCase()} error");
+    if (error.id != errorId) buffer.write(" with request ${error.id}");
+    buffer.write(": ${error.message}");
+    printError(buffer.toString());
     // PROTOCOL error from https://bit.ly/2poTt90
     exitCode = 76; // EX_PROTOCOL
     return error;
   } else {
     var errorMessage = "$error\n${Chain.forTrace(stackTrace)}";
-    stderr.write("Internal compiler error: $errorMessage");
+    printError("Internal compiler error: $errorMessage");
     exitCode = 70; // EX_SOFTWARE
     return ProtocolError()
       ..type = ProtocolErrorType.INTERNAL

lib/src/parse/parser.dart

@@ -651,7 +651,7 @@ class Parser {
     var span = scanner.spanFrom(state);
     return _interpolationMap == null
         ? span
-        : LazyFileSpan(() => _interpolationMap!.mapSpan(span));
+        : LazyFileSpan(() => _interpolationMap.mapSpan(span));
   }
 
   /// Throws an error associated with [span].

lib/src/visitor/async_evaluate.dart

@@ -1812,7 +1812,7 @@ final class _EvaluateVisitor
     if (result != null) {
       isDependency = _inDependency;
     } else {
-      result = await _nodeImporter!.loadAsync(originalUrl, previous, forImport);
+      result = await _nodeImporter.loadAsync(originalUrl, previous, forImport);
       if (result == null) return null;
       isDependency = true;
     }

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 396c8f169d95c601598b8c3be1f4b948ca22effa
+// Checksum: 3986f5db33dd220dcd971a39e8587ca4e52d9a3f
 //
 // ignore_for_file: unused_import
 
@@ -1808,7 +1808,7 @@ final class _EvaluateVisitor
     if (result != null) {
       isDependency = _inDependency;
     } else {
-      result = _nodeImporter!.load(originalUrl, previous, forImport);
+      result = _nodeImporter.load(originalUrl, previous, forImport);
       if (result == null) return null;
       isDependency = true;
     }

pkg/sass_api/pubspec.yaml

@@ -7,7 +7,7 @@ description: Additional APIs for Dart Sass.
 homepage: https://github.com/sass/dart-sass
 
 environment:
-  sdk: ">=3.0.0 <4.0.0"
+  sdk: ">=3.3.0 <4.0.0"
 
 dependencies:
   sass: 1.80.5

pubspec.yaml

@@ -8,7 +8,7 @@ executables:
   sass: sass
 
 environment:
-  sdk: ">=3.0.0 <4.0.0"
+  sdk: ">=3.3.0 <4.0.0"
 
 dependencies:
   args: ^2.0.0

tool/grind.dart

@@ -44,6 +44,8 @@ void main(List<String> args) {
         target: pkg.JSRequireTarget.node, identifier: 'nodeModule'),
     pkg.JSRequire("stream", target: pkg.JSRequireTarget.node),
     pkg.JSRequire("util", target: pkg.JSRequireTarget.node),
+    pkg.JSRequire("./sass-embedded",
+        target: pkg.JSRequireTarget.cli, identifier: 'sass_embedded'),
   ];
   pkg.jsModuleMainLibrary.value = "lib/src/js.dart";
   pkg.npmPackageJson.fn = () =>
@@ -130,6 +132,8 @@ void main(List<String> args) {
 
   afterTask("pkg-npm-dev", _addDefaultExport);
   afterTask("pkg-npm-release", _addDefaultExport);
+  afterTask("pkg-npm-dev", _addOptionalSassEmbedded);
+  afterTask("pkg-npm-release", _addOptionalSassEmbedded);
 
   grind(args);
 }
@@ -302,6 +306,28 @@ function defaultExportDeprecation() {
   File("build/npm/sass.node.mjs").writeAsStringSync(buffer.toString());
 }
 
+/// After building the NPM package, write a wrapper script to lazily
+/// require "sass-embedded/embedded".
+void _addOptionalSassEmbedded() {
+  var buffer = """
+const path = require('path');
+
+module.exports = (function () {
+  try {
+    return require(path.join(path.dirname(require.resolve('sass-embedded')), 'src', 'embedded', 'index.js'));
+  } catch (_) {
+    return {
+      main: function () {
+        console.error('`sass --embedded` requires "sass-embedded" package in pure JS mode.');
+        process.exitCode = 1;
+      }
+    }
+  }
+})();
+""";
+  File("build/npm/sass-embedded.js").writeAsStringSync(buffer);
+}
+
 /// A regular expression to locate the language repo revision in the Dart Sass
 /// Homebrew formula.
 final _homebrewLanguageRegExp = RegExp(