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 e3e5b33aa9823b526aeec448f479829712f6074b..10d0a8c19be4d1d9120b7bb825af451577d32a7a 100644
--- a/pkg/dev_compiler/lib/js/common/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/common/dart_sdk.js
@@ -1864,11 +1864,12 @@
       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();
     });