Turn big_circle.dart into a test

Fixes #829

packages/unit/test/widget/align_test.dart

@@ -4,7 +4,7 @@ import 'package:test/test.dart';
 import 'build_utils.dart';
 
 void main() {
-  test('Global key mutate during iteration', () {
+  test('Align smoke test', () {
     WidgetTester tester = new WidgetTester();
 
     tester.pumpFrame(() {

examples/widgets/big_circle.dart → packages/unit/test/widget/box_decoration_test.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.dart';
 import 'package:sky/theme/colors.dart' as colors;
+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'build_utils.dart';
 
-class BigCircleApp extends App {
-  Widget build() {
+void main() {
+  test('Circles can have uniform borders', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
       return new Container(
         padding: new EdgeDims.all(50.0),
         decoration: new BoxDecoration(
@@ -15,9 +17,7 @@ class BigCircleApp extends App {
           backgroundColor: colors.Teal[600]
         )
       );
-  }
-}
+    });
 
-void main() {
-  runApp(new BigCircleApp());
+  });
 }