Add implementation of Isolate.resolvePackageUri for dart2js.

tests/isolate/browser/package_resolve_browser_hook2_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:js';
+import 'dart:isolate';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+
+main() async {
+  useHtmlConfiguration();
+
+  setUp(() {
+      context['defaultPackagesBase'] = 'path1/';
+  });
+
+  test('hook overrides package-uri resolution', () async {
+    var uri = await Isolate.resolvePackageUri(Uri.parse('package:foo/bar.txt'));
+    expect(uri, Uri.base.resolve('path1/foo/bar.txt'));
+  });
+
+  test('hook is read once, on the first use of resolvePackageUri', () async {
+    await Isolate.resolvePackageUri(Uri.parse('package:foo/bar.txt'));
+    context['defaultPackagesBase'] = 'path2/';
+    var uri = await Isolate.resolvePackageUri(Uri.parse('package:foo/bar.txt'));
+    expect(uri, Uri.base.resolve('path1/foo/bar.txt'));
+  });
+}