Use 'print' to send responses/notifications to the client.

pkg/analysis_server/lib/src/server/stdio_server.dart

 // Copyright (c) 2014, 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 stdio.server;
 
 import 'dart:async';
-import 'dart:io';
 
 import 'package:analysis_server/src/channel/byte_stream_channel.dart';
 import 'package:analysis_server/src/socket_server.dart';
 
 /**
  * Instances of the class [StdioServer] implement a simple server operating
  * over standard input and output. The primary responsibility of this server
  * is to split incoming messages on newlines and pass them along to the
  * analysis server.
  */
 class StdioAnalysisServer {
   /**
    * An object that can handle either a WebSocket connection or a connection
    * to the client over stdio.
    */
   SocketServer socketServer;
 
   /**
    * Initialize a newly created stdio server.
    */
   StdioAnalysisServer(this.socketServer);
 
   /**
    * Begin serving requests over stdio.
    *
    * Return a future that will be completed when stdin closes.
    */
   Future serveStdio() {
-    ByteStreamServerChannel serverChannel = new ByteStreamServerChannel(
-        stdin,
-        stdout.nonBlocking,
-        socketServer.instrumentationService);
+    PrintServerChannel serverChannel =
+        new PrintServerChannel(socketServer.instrumentationService);
     socketServer.createAnalysisServer(serverChannel);
     return serverChannel.closed;
   }
 }