Move all tests using dart:io to a separate directory

tests/standalone/src/io/SocketManyConnectionsTest.dart

 // Copyright (c) 2012, 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.
 
 #import("dart:io");
 #import("dart:isolate");
 #source("TestingServer.dart");
 
@@ skipping 72 matching lines @@
     connection.onClosed = closeHandler;
     connection.onError = errorHandler;
   }
 
   int _connections = 0;
 }
 
 main() {
   SocketManyConnectionsTest test = new SocketManyConnectionsTest.start();
 }