Rename the _fletch_system and service and dart: libraries

lib/fletch/fletch.dart

 // Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 library dart.fletch;
 
-import 'dart:_fletch_system' as fletch;
+import 'dart:fletch._system' as fletch;
 
 /// Fibers are lightweight co-operative multitask units of execution. They
 /// are scheduled on top of OS-level threads, but they are cheap to create
 /// and block.
 class Fiber {
 
   // We keep track of the top of the coroutine stack and
   // the list of other fibers that are waiting for this
   // fiber to exit.
   Coroutine _coroutine;
@@ skipping 463 matching lines @@
 class _ChannelEntry {
   final message;
   final Fiber sender;
   _ChannelEntry next;
   _ChannelEntry(this.message, this.sender);
 }
 
 bool isImmutable(Object object) => _isImmutable(object);
 
 @fletch.native external bool _isImmutable(String string);