Add some basic tests for the material buttons

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();
+}