Use microtasks in dart.async

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
index a88138bab1b07aaf4b95cf1014194da0bdbeb3c1..090ec26b89e0345e41f67fef573ce7c48baf38ef 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
@@ -47,7 +47,7 @@ async_(gen, T, @rest args) => JS('', '''(() => {
     // Chain the Future so `await` receives the Future's value.
     return future.then($dynamic)(onValue, {onError: onError});
   }
-  return ${getGenericClass(Future)}($T).new(function() {
+  return ${getGenericClass(Future)}($T).microtask(function() {
     iter = $gen.apply(null, $args)[Symbol.iterator]();
     return onValue();
   });