Use microtasks in dart.async

pkg/dev_compiler/lib/js/legacy/dart_sdk.js

Index: pkg/dev_compiler/lib/js/legacy/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
index 2b9490006585a396794c4312ae12990484910e0d..cf152c40d3030c101d8d4555c098cd9618a3718a 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
@@ -1907,7 +1907,7 @@ dart_library.library('dart_sdk', null, /* Imports */[
       }
       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();
     });