Add implementation of Isolate.resolvePackageUri for dart2js.

tests/isolate/browser/package_resolve_browser_test.dart

Index: tests/isolate/browser/package_resolve_browser_test.dart
diff --git a/tests/isolate/browser/package_resolve_browser_test.dart b/tests/isolate/browser/package_resolve_browser_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cdb8ba1e785b664114cdccabf42faf432b221976
--- /dev/null
+++ b/tests/isolate/browser/package_resolve_browser_test.dart
@@ -0,0 +1,13 @@
+// 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';
+
+final PACKAGE_URI = "package:foo/bar.txt";
+
+main() async {
+  var uri = await Isolate.resolvePackageUri(Uri.parse(PACKAGE_URI));
+  expect(uri, Uri.base.resolve('packages/foo/bar.txt'));
+}