Reimplement devfs in dart. Implementation now writes to disk.

sdk/lib/vmservice/vmservice_sources.gypi

 # Copyright (c) 2015, the Dart 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 file.
 
 # Sources that make up the library "dart:_vmservice".
 
 {
   'sources': [
     'vmservice.dart',
     # The above file needs to be first as it imports required libraries.
     'asset.dart',
     'client.dart',
     'constants.dart',
+    'devfs.dart',
     'running_isolate.dart',
     'running_isolates.dart',
     'message.dart',
     'message_router.dart',
   ],
 }