Move Barback to a more thoroughly push-based model.

pkg/barback/test/utils.dart

 // 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 barback.test.utils;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert' show Encoding;
 
@@ skipping 80 matching lines @@
   // by MockProvider to exist.
   for (var package in transformers.keys) {
     assetMap.putIfAbsent(package, () => new AssetSet());
   }
 
   _provider = new MockProvider(assetMap);
   _barback = new Barback(_provider);
   _nextBuildResult = 0;
   _nextLog = 0;
 
-  transformers.forEach(_barback.updateTransformers);
+  schedule(() => transformers.forEach(_barback.updateTransformers));
 
   // There should be one successful build after adding all the transformers but
   // before adding any sources.
   if (!transformers.isEmpty) buildShouldSucceed();
 }
 
 /// Updates [assets] in the current [PackageProvider].
 ///
 /// Each item in the list may either be an [AssetId] or a string that can be
 /// parsed as one.
@@ skipping 452 matching lines @@
 
   _MockAsset(this.id, this.contents);
 
   Future<String> readAsString({Encoding encoding}) =>
       new Future.value(contents);
 
   Stream<List<int>> read() => throw new UnimplementedError();
 
   String toString() => "MockAsset $id $contents";
 }