Add a test for the sector app.

This also moves the test for stocks into a new tests/examples/
directory, where we can put tests that test the examples.

TBR=abarth

Review URL: https://codereview.chromium.org/1182223004.

examples/widgets/sector.dart

@@ -28,7 +28,7 @@ RenderBox initCircle() {
 class SectorApp extends App {
 
   RenderBoxToRenderSectorAdapter sectors = initCircle();
-  math.Random rand = new math.Random();
+  math.Random rand = new math.Random(1);
 
   void addSector() {
     double deltaTheta;

tests/examples/sector.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 'dart:async';
+
+import 'package:sky/app/view.dart';
+import 'package:sky/widgets/widget.dart';
+
+import '../../examples/widgets/sector.dart';
+import '../resources/display_list.dart';
+
+main() async {
+  TestRenderView testRenderView = new TestRenderView();
+  SectorApp app = new SectorApp();
+  runApp(app, renderViewOverride: testRenderView);
+  await testRenderView.checkFrame();
+  app.addSector();
+  await testRenderView.checkFrame();
+  app.addSector();
+  await testRenderView.checkFrame();
+  app.addSector();
+  await testRenderView.checkFrame();
+  app.removeSector();
+  await testRenderView.checkFrame();
+  app.removeSector();
+  await testRenderView.checkFrame();
+  app.addSector();
+  await testRenderView.checkFrame();
+  testRenderView.endTest();
+}