Move all tests using dart:io to a separate directory

tests/standalone/src/test_extension.dart

Index: tests/standalone/src/test_extension.dart
diff --git a/tests/standalone/src/test_extension.dart b/tests/standalone/src/test_extension.dart
deleted file mode 100644
index e36d3eb446dedf933e9879e4c07dda3561c5c934..0000000000000000000000000000000000000000
--- a/tests/standalone/src/test_extension.dart
+++ /dev/null
@@ -1,18 +0,0 @@
-// 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.
-
-#library("test_extension");
-
-#import("dart-ext:test_extension");
-
-class Cat {
-  Cat(this.x);
-
-  num x;
-
-  String toString() => 'cat $x';
-
-  // Implements (a != null) ? a : b using a native C++ function and the API.
-  static int ifNull(a, b) native 'Cat_IfNull';
-}