Don't zone-register async callbacks for every await call in the VM.

sdk/lib/async/future_impl.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.async;
 
 /** The onValue and onError handlers return either a value or a future */
 typedef dynamic _FutureOnValue<T>(T value);
 /** Test used by [Future.catchError] to handle skip some errors. */
 typedef bool _FutureErrorTest(var error);
@@ skipping 157 matching lines @@
    * will complete with the same result.
    * All listeners are forwarded to the other future.
    *
    * The cases are disjoint - incomplete and unchained ([_INCOMPLETE]),
    * incomplete and chained ([_CHAINED]), or completed with value or error
    * ([_VALUE] or [_ERROR]) - so the field only needs to hold
    * one value at a time.
    */
   var _resultOrListeners;
 
+  // This constructor is used by async/await.
   _Future();
 
   /// Valid types for value: `T` or `Future<T>`.
   _Future.immediate(value) {
     _asyncComplete(value);
   }
 
   _Future.immediateError(var error, [StackTrace stackTrace]) {
     _asyncCompleteError(error, stackTrace);
   }
@@ skipping 18 matching lines @@
     Zone currentZone = Zone.current;
     if (!identical(currentZone, _ROOT_ZONE)) {
       f = currentZone.registerUnaryCallback(f);
       if (onError != null) {
         onError = _registerErrorHandler(onError, currentZone);
       }
     }
     return _thenNoZoneRegistration(f, onError);
   }
 
-  // This function is used by async/await.
+  // This method is used by async/await.
   Future _thenNoZoneRegistration(f(T value), Function onError) {
     _Future result = new _Future();
     _addListener(new _FutureListener.then(result, f, onError));
     return result;
   }
 
   Future catchError(Function onError, { bool test(error) }) {
     _Future result = new _Future();
     if (!identical(result._zone, _ROOT_ZONE)) {
       onError = _registerErrorHandler(onError, result._zone);
@@ skipping 22 matching lines @@
   T get _value {
     assert(_isComplete && _hasValue);
     return _resultOrListeners;
   }
 
   AsyncError get _error {
     assert(_isComplete && _hasError);
     return _resultOrListeners;
   }
 
+  // This method is used by async/await.
   void _setValue(T value) {
     assert(!_isComplete);  // But may have a completion pending.
     _state = _VALUE;
     _resultOrListeners = value;
   }
 
   void _setErrorObject(AsyncError error) {
     assert(!_isComplete);  // But may have a completion pending.
     _state = _ERROR;
     _resultOrListeners = error;
@@ skipping 389 matching lines @@
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }