Clean up the tests to be more maintainable.

sky/tests/widgets/buttons.dart

 // Copyright 2015 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 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/flat_button.dart';
 import 'package:sky/widgets/floating_action_button.dart';
 import 'package:sky/widgets/raised_button.dart';
 
 import '../resources/display_list.dart';
 
 main() async {
   WidgetTester tester = new WidgetTester();
 
   await tester.test(() {
     return new Center(child: new RaisedButton(child: new Text("ENGAGE")));
   });
 
   await tester.test(() {
     return new Center(child: new FlatButton(child: new Text("ENGAGE")));
   });
 
   await tester.test(() {
     return new Center(child: new FloatingActionButton(child: new Text("+")));
   });
 
-  await tester.endTest();
+  tester.endTest();
 }