Move all tests using dart:io to a separate directory

tests/standalone/src/io/SocketExceptionTest.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.
 //
 // Tests socket exceptions.
 
 #import("dart:io");
 
 class SocketExceptionTest {
 
@@ skipping 160 matching lines @@
   static void testMain() {
     serverSocketExceptionTest();
     clientSocketExceptionTest();
     indexOutOfRangeExceptionTest();
   }
 }
 
 main() {
   SocketExceptionTest.testMain();
 }