Add implementation of Isolate.resolvePackageUri for dart2js.

tests/isolate/browser/package_resolve_browser_hook_test.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.
+
+import 'dart:isolate';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+
+main() async {
+  useHtmlConfiguration();
+
+  test('defaultPackagesBase hook overrides package-uri resolution', () async {
+    var uri = await Isolate.resolvePackageUri(Uri.parse('package:foo/bar.txt'));
+    expect(uri, Uri.base.resolve('path/set/from/hook/foo/bar.txt'));
+  });
+}