Implement spawnFunction from the new isolate api.

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();
   }
 
   factory ReceivePort.singleShot() {
@@ skipping 137 matching lines @@
   }
 
   // Starts a new isolate calling the run method on a new instance of the
   // remote class's type.
   // Returns the send port which is passed to the newly created isolate.
   // This method is being dispatched to from the public core library code.
   static SendPort _start(Isolate isolate, bool light)
       native "IsolateNatives_start";
 }
 
-// TODO(sigmund,turnidge): implement
+_getPortInternal() native "isolate_getPortInternal";
 
-SendPort _spawnFunction(void topLevelFunction()) {
-  throw new NotImplementedException();
+ReceivePort _portInternal;
+
+ReceivePort get _port() {
+  if (_portInternal) {
+    return _portInternal;
+  }
+  _portInternal = _getPortInternal();
+  return _portInternal;
 }
 
+_spawnFunction(void topLevelFunction()) native "isolate_spawnFunction";
+
+// TODO(sigmund,turnidge): implement
 SendPort _spawnUri(String uri) {
   throw new NotImplementedException();
 }
-
-ReceivePort _port = null;