Fix pkg/analyzer/tool/task_dependency_graph/check_test.

pkg/analyzer/tool/task_dependency_graph/generate.dart

Index: pkg/analyzer/tool/task_dependency_graph/generate.dart
diff --git a/pkg/analyzer/tool/task_dependency_graph/generate.dart b/pkg/analyzer/tool/task_dependency_graph/generate.dart
index db2cd09ddca72a07fe114162fdbbb7b843a7f7cf..068790f5787d3e45e9c50f3d6ca8ed47e5a4978c 100644
--- a/pkg/analyzer/tool/task_dependency_graph/generate.dart
+++ b/pkg/analyzer/tool/task_dependency_graph/generate.dart
@@ -154,14 +154,15 @@ ${generateGraphData()}
     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,