Rename the _fletch_system and service and dart: libraries

lib/os/os.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.os;
 
-import 'dart:_fletch_system' as fletch;
+import 'dart:fletch._system' as fletch;
 import 'dart:fletch';
 import 'dart:fletch.ffi';
 
 part 'native_process.dart';
 part 'event_handler.dart';
 part 'event_handler_linux.dart';
 part 'event_handler_macos.dart';
 
 final ForeignFunction _nanosleep = ForeignLibrary.main.lookup("nanosleep");
 
@@ skipping 11 matching lines @@
   timespec.tv_sec = milliseconds ~/ 1000;
   timespec.tv_nsec = (milliseconds % 1000) * 1000000;
   int result;
   try {
     result = _nanosleep.icall$2Retry(timespec, timespec);
   } finally {
     timespec.free();
   }
   if (result != 0) throw "Failed to call 'nanosleep': ${Foreign.errno}";
 }