Rename several classes in barback.

pkg/barback/test/package_graph/errors_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("errors if two transformers output the same file", () {
     initGraph(["app|foo.a"], {"app": [
       [
@@ skipping 146 matching lines @@
     schedule(() {
       updateSources(["pkg1|foo.txt", "pkg2|foo.txt"]);
     });
 
     buildShouldFail([
       equals(BadTransformer.ERROR),
       equals(BadTransformer.ERROR)
     ]);
   });
 }