Split dart:builtin into dart:builtin and dart:io.

tests/standalone/src/EchoServerTest.dart

 // Copyright (c) 2011, 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.
 //
 // Echo server test program for testing sockets.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 #library("EchoServerTest");
+#import("dart:io");
 #source("TestingServer.dart");
 
 class EchoServerTest {
 
   static void testMain() {
     EchoServerGame echoServerGame = new EchoServerGame.start();
   }
 }
 
 class EchoServerGame {
@@ skipping 165 matching lines @@
 
     connection.dataHandler = messageHandler;
     connection.closeHandler = closeHandler;
     connection.errorHandler = errorHandler;
   }
 }
 
 main() {
   EchoServerTest.testMain();
 }