dart2js: Don't zone-register callbacks in async functions for every await.

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 185 matching lines @@
     if (value) {
       assert(!_isComplete);
       _state = _CHAINED;
     } else {
       assert(_isChained);
       _state = _INCOMPLETE;
     }
   }
 
   Future then(f(T value), { Function onError }) {
-    _Future result = new _Future();
-    if (!identical(result._zone, _ROOT_ZONE)) {
-      f = result._zone.registerUnaryCallback(f);
+    Zone currentZone = Zone.current;
+    if (!identical(currentZone, _ROOT_ZONE)) {
+      f = currentZone.registerUnaryCallback(f);
       if (onError != null) {
-        onError = _registerErrorHandler(onError, result._zone);
+        onError = _registerErrorHandler(onError, currentZone);
       }
     }
+    return _thenNoZoneRegistration(f, onError);
+  }
+
+  // This function 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);
       if (test != null) test = result._zone.registerUnaryCallback(test);
     }
@@ skipping 429 matching lines @@
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }