Move all tests using dart:io to a separate directory

tests/standalone/src/ManyEchoServerTest.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.
-// Stress test isolate generation.
-
-#library("ManyEchoServerTest.dart");
-#import("EchoServerTest.dart", prefix: "single");
-
-class ManyEchoServerTest {
-  static testMain() {
-    for (int i = 0; i < 5000; i++) {
-      single.EchoServerTest.testMain();
-    }
-  }
-}
-
-main() {
-  ManyEchoServerTest.testMain();
-}