Fix task model grapher to use package root.

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 120 matching lines @@
   }
 
   /**
    * Generate the task dependency graph and return it as a [String].
    */
   String generateFileContents() {
     List<String> lines = <String>[];
     resourceProvider = PhysicalResourceProvider.INSTANCE;
     DartSdk sdk = DirectoryBasedDartSdk.defaultSdk;
     context = AnalysisEngine.instance.createAnalysisContext();
-    JavaFile packagesDir = new JavaFile(path.join(rootDir, 'packages'));
+    String packageRootPath;
+    if (Platform.packageRoot.isNotEmpty) {
+      packageRootPath = Platform.packageRoot;
+    } else {
+      packageRootPath = path.join(rootDir, 'packages');
+    }
+    JavaFile packagesDir = new JavaFile(packageRootPath);
     List<UriResolver> uriResolvers = [
       new DartUriResolver(sdk),
       new PackageUriResolver(<JavaFile>[packagesDir]),
       new FileUriResolver()
     ];
     context.sourceFactory = new SourceFactory(uriResolvers);
     Source dartDartSource =
         setupSource(path.join('lib', 'src', 'task', 'dart.dart'));
     Source taskSource = setupSource(path.join('lib', 'plugin', 'task.dart'));
     Source modelSource = setupSource(path.join('lib', 'task', 'model.dart'));
@@ skipping 179 matching lines @@
   @override
   visitIdentifier(Identifier node) {
     Element element = node.staticElement;
     if (element is PropertyAccessorElement &&
         element.isGetter &&
         element.returnType.isSubtypeOf(type)) {
       callback(element);
     }
   }
 }