Move all tests using dart:io to a separate directory

tests/standalone/src/ProcessStartExceptionTest.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.
-
-#import("dart:io");
-
-class ProcessStartExceptionTest {
-
-  static void testStartError() {
-    Process process =
-        new Process.start("__path_to_something_that_should_not_exist__",
-                          const []);
-
-    process.onExit = (int exitCode) {
-      Expect.fail("exit handler called");
-    };
-
-    process.onError = (ProcessException e) {
-      Expect.equals(2, e.errorCode);
-    };
-  }
-}
-
-main() {
-  ProcessStartExceptionTest.testStartError();
-}