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 f056e58b355242a8afc6ec59c88a49505e4949b5..c15962df0f4d6840708f81c74b89cae006b26921 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_sdk.js
@@ -1865,11 +1865,12 @@ dart_library.library('dart_sdk', null, /* Imports */[
       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();
     });