Use microtasks in dart.async

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.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.
 
 /// This library adapts ES6 generators to implement Dart's async/await.
 /// It's designed to interact with Dart's Future/Stream and follow Dart
 /// async/await semantics.
 /// See https://github.com/dart-lang/sdk/issues/27315 for ideas on
 /// reconciling Dart's Future and ES6 Promise.
 /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
@@ skipping 29 matching lines @@
   function next(ret) {
     if (ret.done) return ret.value;
     // Checks if the awaited value is a Future.
     let future = ret.value;
     if (!$instanceOf(future, ${getGenericClass(Future)})) {
       future = $Future.value(future);
     }
     // Chain the Future so `await` receives the Future's value.
     return future.then($dynamic)(onValue, {onError: onError});
   }
-  return ${getGenericClass(Future)}($T).new(function() {
+  return ${getGenericClass(Future)}($T).microtask(function() {
     iter = $gen.apply(null, $args)[Symbol.iterator]();
     return onValue();
   });
 })()''');
 
 // Implementation inspired by _AsyncStarStreamController in
 // dart-lang/sdk's runtime/lib/core_patch.dart
 //
 // Given input like:
 //
@@ skipping 154 matching lines @@
       this.controller.addError(error, stackTrace);
     }
   }
 ''');
 
 /// Returns a Stream of T implemented by an async* function. */
 ///
 asyncStar(gen, T, @rest args) => JS('', '''(() => {
   return new $_AsyncStarStreamController($gen, $T, $args).controller.stream;
 })()''');