Add a hacky hook to override the loader behavior.

lib/src/runner/hack_load_vm_file_hook.dart

+// Copyright (c) 2015, 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.runner.hack_load_vm_file_hook;
+
+typedef Future<RunnerSuite> LoadVMFileHook(String path, Metadata metadata,
+    Configuration config);
+
+/// **Do not set or use this function without express permission from the test
+/// package authors**.
+///
+/// A function that overrides the loader's default behavior for loading test
+/// suites on the Dart VM. This function takes the path to the file, the
+/// file's metadata, and the test runner's configuration and returns a
+/// [RunnerSuite] for that file.
+LoadVMFileHook loadVMFileHook;