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

sdk/lib/io/common.dart

Index: sdk/lib/io/common.dart
diff --git a/sdk/lib/io/common.dart b/sdk/lib/io/common.dart
index 5f1700fc0874785dda8b2c437ffde0a97f12b270..3f45419a54c18f21403ff94f1615e42766e1b912 100644
--- a/sdk/lib/io/common.dart
+++ b/sdk/lib/io/common.dart
@@ -123,3 +123,18 @@ _BufferAndStart _ensureFastAndSerializableByteData(
 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);
+}