Fix pkg/analyzer/tool/task_dependency_graph/check_test.

pkg/analyzer/tool/task_dependency_graph/generate.dart

 // Copyright (c) 2015, 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.
 
 /**
  * This file contains code to output a description of tasks and their
  * dependencies in ".dot" format.  Prior to running, the user should run "pub
  * get" in the analyzer directory to ensure that a "packages" folder exists.
  *
  * TODO(paulberry):
@@ skipping 136 matching lines @@
   String generateGraphData() {
     if (!hasInitializedPlugins) {
       AnalysisEngine.instance.processRequiredPlugins();
       hasInitializedPlugins = true;
     }
     List<String> lines = <String>[];
     resourceProvider = PhysicalResourceProvider.INSTANCE;
     DartSdk sdk = new FolderBasedDartSdk(resourceProvider,
         FolderBasedDartSdk.defaultSdkDirectory(resourceProvider));
     context = AnalysisEngine.instance.createAnalysisContext();
-    String packageRootPath;
+    ContextBuilder builder = new ContextBuilder(resourceProvider, null, null);
     if (Platform.packageRoot != null) {
-      packageRootPath = Uri.parse(Platform.packageRoot).toFilePath();
+      builder.defaultPackagesDirectoryPath = Uri.parse(Platform.packageRoot).toFilePath();
+    } else if (Platform.packageConfig != null) {
+      builder.defaultPackageFilePath = Platform.packageConfig;
     } else {
-      packageRootPath = path.join(rootDir, 'packages');
+      // Let the context builder use the default algorithm for package
+      // resolution.
     }
-    ContextBuilder builder = new ContextBuilder(resourceProvider, null, null);
-    builder.defaultPackagesDirectoryPath = packageRootPath;
     List<UriResolver> uriResolvers = [
       new DartUriResolver(sdk),
       new PackageMapUriResolver(resourceProvider,
           builder.convertPackagesToMap(builder.createPackageMap(''))),
       new ResourceUriResolver(PhysicalResourceProvider.INSTANCE)
     ];
     context.sourceFactory = new SourceFactory(uriResolvers);
     Source dartDartSource =
         setupSource(path.join('lib', 'src', 'task', 'dart.dart'));
     Source taskSource = setupSource(path.join('lib', 'plugin', 'task.dart'));
@@ skipping 178 matching lines @@
   @override
   visitIdentifier(Identifier node) {
     Element element = node.staticElement;
     if (element is PropertyAccessorElement &&
         element.isGetter &&
         element.returnType.isSubtypeOf(type)) {
       callback(element);
     }
   }
 }