Properly split dart:_vmservice across sdk and runtime

runtime/lib/vmservice/client.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;
-
-// A service client.
-abstract class Client {
-  final VMService service;
-  final bool sendEvents;
-  final Set<String> streams = new Set<String>();
-
-  Client(this.service, { bool sendEvents: true })
-      : this.sendEvents = sendEvents {
-    service._addClient(this);
-  }
-
-  // Disconnects the client.
-  disconnect();
-
-  /// When implementing, call [close] when the network connection closes.
-  void close() {
-    service._removeClient(this);
-  }
-
-  /// Call to process a message. Response will be posted with 'seq'.
-  void onMessage(var seq, Message message) {
-    try {
-      // Send message to service.
-      service.route(message).then((response) {
-        // Call post when the response arrives.
-        post(response);
-      });
-    } catch (e, st) {
-      message.setErrorResponse(
-          kInternalError, 'Unexpected exception:$e\n$st');
-      post(message.response);
-    }
-  }
-
-  // Sends a result to the client.  Implemented in subclasses.
-  void post(dynamic result);
-
-  dynamic toJson() {
-    return {
-    };
-  }
-}