Implement spawnUri from dart:isolate. This function allows us to

tests/isolate/v2_spawn_uri_vm_test.dart

Index: tests/isolate/v2_spawn_uri_vm_test.dart
===================================================================
--- tests/isolate/v2_spawn_uri_vm_test.dart	(revision 0)
+++ tests/isolate/v2_spawn_uri_vm_test.dart	(revision 0)
@@ -0,0 +1,24 @@
+// Copyright (c) 2012, 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.
+
+// Example of spawning an isolate from a URI
+// Note: the following comment is used by test.dart to additionally compile the
+// other isolate's code.
+// OtherScripts=v2_spawn_uri_child_isolate.dart
+#library('spawn_tests');
+#import('../../lib/unittest/unittest.dart');
+#import('dart:isolate');
+
+main() {
+  test('isolate fromUri - send and reply', () {
+    ReceivePort port = new ReceivePort();
+    port.receive(expectAsync2((msg, _) {
+      expect(msg).equals('re: hi');
+      port.close();
+    }));
+
+    SendPort s = spawnUri('v2_spawn_uri_child_isolate.dart');
+    s.send('hi', port.toSendPort());
+  });
+}