[await sync-async flag] Add test of sync async execution (#16795)

packages/flutter/test/engine/dart_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:test/test.dart';
+
+/// Verifies Dart semantics governed by flags set by Flutter tooling.
+void main() {
+  group('Async', () {
+    String greeting = 'hello';
+    Future<void> changeGreeting() async {
+      greeting += ' 1';
+      await new Future<void>.value(null);
+      greeting += ' 2';
+    }
+    test('execution of async method starts synchronously', () async {
+      expect(greeting, 'hello');
+      final Future<void> future = changeGreeting();
+      expect(greeting, 'hello 1');
+      await future;
+      expect(greeting, 'hello 1 2');
+    });
+  });
+}