Use package:sky imports consistently

sky/sdk/lib/widgets/navigator.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 'basic.dart';
+import 'package:sky/widgets/basic.dart';
 
 typedef Widget Builder(Navigator navigator, RouteBase route);
 
 abstract class RouteBase {
   RouteBase({ this.name });
   final String name;
   Widget build(Navigator navigator, RouteBase route);
   void popState() { }
 }
 
@@ skipping 94 matching lines @@
   void pop() {
     setState(() {
       state.pop();
     });
   }
 
   Widget build() {
     return state.currentRoute.build(this, state.currentRoute);
   }
 }