Rename several classes in barback.

pkg/barback/test/package_graph/source_test.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.asset_graph.source_test;
+library barback.test.package_graph.source_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
-import 'package:barback/src/asset_graph.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
 main() {
   initConfig();
   test("gets a source asset", () {
     initGraph(["app|foo.txt"]);
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt");
@@ skipping 112 matching lines @@
     resumeProvider();
 
     buildShouldSucceed();
     waitForBuild();
 
     schedule(() {
       expect(transformer.numRuns, equals(2));
     });
   });
 }