Move all tests using dart:io to a separate directory

tests/standalone/src/io/EchoServerTest.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.
 //
 // Echo server test program for testing sockets.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
@@ skipping 178 matching lines @@
 
     connection.onData = messageHandler;
     connection.onClosed = closeHandler;
     connection.onError = errorHandler;
   }
 }
 
 main() {
   EchoServerTest.testMain();
 }