Fix an analyzer error in barback.

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:convert' show Encoding;
 
 import 'package:barback/barback.dart';
@@ skipping 542 matching lines @@
     var id = new AssetId.parse(name);
     _errors.remove(id);
     _syncErrors.remove(id);
     (_assets[id.package][id] as _MockAsset).contents = contents;
   }
 
   void _setAssetError(String name, bool async) {
     (async ? _errors : _syncErrors).add(new AssetId.parse(name));
   }
 
-  List<AssetId> getAllAssets(String package) =>
-      _assets[package].map((asset) => asset.id);
+  Stream<AssetId> getAllAssetIds(String package) =>
+      new Stream.fromIterable(_assets[package].map((asset) => asset.id));
 
   Future<Asset> getAsset(AssetId id) {
     // Eagerly load the asset so we can test an asset's value changing between
     // when a load starts and when it finishes.
     var assets = _assets[id.package];
     var asset;
     if (assets != null) asset = assets[id];
 
     if (_syncErrors.contains(id)) throw new MockLoadException(id);
     var hasError = _errors.contains(id);
@@ skipping 29 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";
 }