Use microtasks in dart.async

pkg/dev_compiler/lib/js/common/dart_sdk.js

Index: pkg/dev_compiler/lib/js/common/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/common/dart_sdk.js b/pkg/dev_compiler/lib/js/common/dart_sdk.js
index cf09b7c64a4dd5acfe31b7e6a34a56abb394009b..4e933b06672e03dd650b1914d30cbfb68e49950f 100644
--- a/pkg/dev_compiler/lib/js/common/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/common/dart_sdk.js
@@ -1906,7 +1906,7 @@
       }
       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();
     });