Change the DevFS base path

runtime/bin/vmservice/vmservice_io.dart

 // Copyright (c) 2013, 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.
 
 library vmservice_io;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 47 matching lines @@
   if (_registerSignalHandlerTimer != null) {
     _registerSignalHandlerTimer.cancel();
     _registerSignalHandlerTimer = null;
   }
   // Call out to embedder's shutdown callback.
   _shutdown();
 }
 
 Future<Uri> createTempDirCallback(String base) async {
   Directory temp = await Directory.systemTemp.createTemp(base);
-  return temp.uri;
+  // Underneath the temporary directory, create a directory with the
+  // same name as the DevFS name [base].
+  var fsUri = temp.uri.resolveUri(new Uri.directory(base));
+  await new Directory.fromUri(fsUri).create();
+  return fsUri;
 }
 
 Future deleteDirCallback(Uri path) async {
   Directory dir = new Directory.fromUri(path);
   await dir.delete(recursive: true);
 }
 
 class PendingWrite {
   PendingWrite(this.uri, this.bytes);
   final Completer completer = new Completer();
@@ skipping 159 matching lines @@
     Timer.run(() {});
   }
   scriptLoadPort.handler = _processLoadRequest;
   // Register signal handler after a small delay to avoid stalling main
   // isolate startup.
   _registerSignalHandlerTimer = new Timer(shortDelay, _registerSignalHandler);
   return scriptLoadPort;
 }
 
 _shutdown() native "VMServiceIO_Shutdown";