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

tests/standalone/src/SocketCloseTest.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 socket close events.
 
+#import("dart:io");
 
 final SERVERSHUTDOWN = -1;
 final ITERATIONS = 10;
 
 
 class SocketClose {
 
   SocketClose.start(mode)
       : _receivePort = new ReceivePort(),
         _sendPort = null,
@@ skipping 308 matching lines @@
   // 5: Client sends. Server responds and half closes.
   // 6: Client sends and half-closes. Server responds and half closes.
   new SocketClose.start(0);
   new SocketClose.start(1);
   new SocketClose.start(2);
   new SocketClose.start(3);
   new SocketClose.start(4);
   new SocketClose.start(5);
   new SocketClose.start(6);
 }