Properly split dart:_vmservice across sdk and runtime

runtime/lib/vmservice/running_isolate.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.
-
-part of vmservice;
-
-class RunningIsolate implements MessageRouter {
-  final int portId;
-  final SendPort sendPort;
-  final String name;
-
-  RunningIsolate(this.portId, this.sendPort, this.name);
-
-  String get serviceId => 'isolates/$portId';
-
-  Future<String> route(Message message) {
-    // Send message to isolate.
-    return message.send(sendPort);
-  }
-}