Use microtasks in dart.async

pkg/dev_compiler/lib/js/es6/dart_sdk.js

Index: pkg/dev_compiler/lib/js/es6/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/es6/dart_sdk.js b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
index 5f47f0bd058f4184f61ccafb151dc4fa250ba648..69f6bdaf152affabf4f78f1e1c96645b1f1bf5e8 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -1862,11 +1862,12 @@ dart.async = function(gen, T, ...args) {
     if (ret.done) return ret.value;
     let future = ret.value;
     if (!dart.is(future, dart.getGenericClass(async.Future))) {
-      future = async.Future.value(future);
+      var result = future;
+      future = async.Future.microtask(() => result);
     }
     return future.then(dart.dynamic)(onValue, {onError: onError});
   }
-  return dart.getGenericClass(async.Future)(T).new(function() {
+  return dart.getGenericClass(async.Future)(T).microtask(function() {
     iter = gen.apply(null, args)[Symbol.iterator]();
     return onValue();
   });