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

runtime/lib/async_patch.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";
 
+// We need to pass the value as first argument and leave the second and third
+// arguments empty (used for error handling).
+// See vm/ast_transformer.cc for usage.
+Function _asyncThenWrapperHelper(continuation) {
+  return Zone.current.registerUnaryCallback((x) => continuation(x, null, null));
+}
+
+// We need to pass the exception and stack trace objects as second and third
+// parameter to the continuation.  See vm/ast_transformer.cc for usage.
+Function _asyncErrorWrapperHelper(continuation) {
+  return Zone.current.registerBinaryCallback(
+      (e, s) => continuation(null, e, s));
+}
+
+Future _awaitHelper(
+    var future, Function thenCallback, Function errorCallback) {

[Lasse Reichstein Nielsen, 2015/08/06 14:19:58]

remove the "var", rename "future" to "result" (it might not be a future).

[floitsch, 2015/08/11 15:27:50]

changed to 'expression'. "Result" would be the result of this function.

[floitsch, 2015/08/11 15:28:37]

Actually changed to "object".

+  if (future is! Future) {
+    future = new Future.value(future);
+  }

[Lasse Reichstein Nielsen, 2015/08/06 14:19:58]

  if (result is! Future) {
    result = new _Future().._setValue(result);  // avoid more tests. 
  } else if (result is! _Future) {
    return result.then(thenCallback, onError: errorCallback);
  }
  return result._thenNoZoneRegistration(thenCallback, errorCallback);
}

This avoids one extra test when we already know that we just created a _Future.

[floitsch, 2015/08/11 15:27:50]

Done.

+  if (future is _Future) {
+    return future._thenNoZoneRegistration(thenCallback, errorCallback);
+  } else {
+    return future.then(thenCallback, onError: errorCallback);
+  }
+}