Rename the _fletch_system and service and dart: libraries

lib/core/core_patch.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.core_patch;
 
-import 'dart:_fletch_system' as fletch;
+import 'dart:fletch._system' as fletch;
 
 part 'bigint.dart';
 part 'case.dart';
 part 'double.dart';
 part 'int.dart';
 part 'regexp.dart';
 part 'string.dart';
 
 const patch = "patch";
 
@@ skipping 639 matching lines @@
   @patch static Uri get base {
     return new Uri.file(_base());
   }
 
   @patch static bool get _isWindows => false;
 
   @fletch.native static String _base() {
     throw new RangeError("The Uri.base path is too large");
   }
 }