Random cleanup of various Dart things in our tree.

sky/sdk/example/rendering/shadowed_box.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:sky';
 
 import 'package:sky/rendering/box.dart';
-import 'package:sky/rendering/flex.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/theme/colors.dart';
 import 'package:sky/theme/shadows.dart';
 
 void main() {
   var coloredBox = new RenderDecoratedBox(
     decoration: new BoxDecoration(
       gradient: new RadialGradient(
         center: Point.origin, radius: 500.0,
         colors: [Yellow[500], Blue[500]]),
       boxShadow: shadows[3])
   );
   var paddedBox = new RenderPadding(
     padding: const EdgeDims.all(50.0),
     child: coloredBox);
   new SkyBinding(root: new RenderDecoratedBox(
     decoration: const BoxDecoration(
       backgroundColor: const Color(0xFFFFFFFF)
     ),
     child: paddedBox
   ));
 }