Support dart2js in pub serve.

sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart_forwarding_transformer.dart
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+// Copyright (c) 2013, 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 pub.dart_forwarding_transformer;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import '../utils.dart';
+
+/// A single transformer that just forwards any ".dart" file as an output.
+///
+/// Since the [Dart2JSTransformer] consumes its inputs, this is used in
+/// parallel to make sure the original Dart file is still available for use by
+/// Dartium.
+class DartForwardingTransformer extends Transformer {
+  String get allowedExtensions => ".dart";
+
+  Future apply(Transform transform) {
+    return newFuture(() {
+      transform.addOutput(transform.primaryInput);
+    });
+  }
+}