Add Dart_PostPriority function to dart_api, and use it in runAsync to schedule immediate events in …

sdk/lib/io/common.dart

 // 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.
 
 part of dart.io;
 
 // Constants used when working with native ports.
 // These must match the constants in runtime/bin/dartutils.h class CObject.
 const int _SUCCESS_RESPONSE = 0;
 const int _ILLEGAL_ARGUMENT_RESPONSE = 1;
@@ skipping 105 matching lines @@
     newBuffer[i] = value;
     j++;
   }
   return new _BufferAndStart(newBuffer, 0);
 }
 
 
 class _IOCrypto {
   external static Uint8List getRandomBytes(int count);
 }
+
+_getEnqueueImmediateFactoryClosure() {
+  return (void callback()) {
+    var port = new ReceivePort();
+    port.receive((msg, reply) {
+      port.close();
+      callback();
+    });
+    _DartAPI._sendPriority(port, null);
+  };
+}
+
+class _DartAPI {
+  external static void _sendPriority(port, msg);
+}