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

tests/standalone/src/SocketManyConnectionsTest.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.
 //
 // Test creating a large number of socket connections.
 
 #library("SocketManyConnectionsTest");
+#import("dart:io");
 #source("TestingServer.dart");
 
 final CONNECTIONS = 200;
 
 class SocketManyConnectionsTest {
 
   SocketManyConnectionsTest.start()
       : _receivePort = new ReceivePort(),
         _sendPort = null,
         _connections = 0,
@@ skipping 64 matching lines @@
     connection.closeHandler = closeHandler;
     connection.errorHandler = errorHandler;
   }
 
   int _connections = 0;
 }
 
 main() {
   SocketManyConnectionsTest test = new SocketManyConnectionsTest.start();
 }