Use microtasks in dart.async

pkg/dev_compiler/lib/js/amd/dart_sdk.js

Index: pkg/dev_compiler/lib/js/amd/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/amd/dart_sdk.js b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
index 06b7a44059cd41b105dc1a4cc4ad0bf1625f7040..43e7c7a33aff6fbc4b85af98464466f346622837 100644
--- a/pkg/dev_compiler/lib/js/amd/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
@@ -1906,7 +1906,7 @@ define([], function() {
       }
       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();
     });