Move all tests using dart:io to a separate directory

tests/standalone/src/ProcessExitTest.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.
-//
-// Process test program to test process communication.
-
-#library("ProcessExitTest");
-#import("dart:io");
-#source("ProcessTestUtil.dart");
-
-class ProcessExitTest {
-  static void testExit() {
-    Process process = new Process.start(getProcessTestFileName(),
-                                        const ["0", "0", "99", "0"]);
-
-    process.onExit = (int exitCode) {
-      Expect.equals(exitCode, 99);
-      process.close();
-    };
-  }
-}
-
-main() {
-  ProcessExitTest.testExit();
-}