Rename the _fletch_system and service and dart: libraries

lib/ffi/ffi.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.
 
 // The dart:fletch.ffi library is a low-level 'foreign function interface'
 // library that allows Dart code to call arbitrary native platform
 // code defined outside the VM.
 library dart.fletch.ffi;
 
-import 'dart:_fletch_system' as fletch;
+import 'dart:fletch._system' as fletch;
 import 'dart:fletch';
 import 'dart:typed_data';
 
 part 'utf.dart';
 
 abstract class Foreign {
   int get address;
 
   const Foreign();
 
@@ skipping 606 matching lines @@
 
   String toString() {
     // Don't include the '\0' character in the encoded string.
     var encodedString = new List(length - 1);
     for (int i = 0; i < length - 1; ++i) {
       encodedString[i] = getUint8(i);
     }
     return _decodeUtf8(encodedString);
   }
 }