Add support for emitting asynchronous timeline events from Dart code

runtime/lib/timeline.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:_internal';
 
 patch int _getTraceClock() native "Timeline_getTraceClock";
 
+patch int _getNextAsyncId() native "Timeline_getNextAsyncId";
+
+patch void _reportTaskEvent(
+    int start,
+    int taskId,
+    String phase,
+    String category,
+    String name,
+    String argumentsAsJson) native "Timeline_reportTaskEvent";
+
 patch void _reportCompleteEvent(
     int start,
     int end,
     String category,
     String name,
     String argumentsAsJson) native "Timeline_reportCompleteEvent";