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

runtime/lib/isolate.dart

 // Copyright (c) 2011, 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.
 
 class _ReceivePortFactory {
   factory ReceivePort() {
     return new _ReceivePortImpl();
   }
 }
 
@@ skipping 125 matching lines @@
 
 ReceivePort get _port() {
   if (_portInternal === null) {
     _portInternal = _getPortInternal();
   }
   return _portInternal;
 }
 
 _spawnFunction(void topLevelFunction()) native "isolate_spawnFunction";
 
-// TODO(sigmund,turnidge): implement
-SendPort _spawnUri(String uri) {
-  throw new NotImplementedException();
-}
+_spawnUri(String uri) native "isolate_spawnUri";