Add a hacky hook to override the loader behavior.

lib/src/runner/loader.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.loader;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:async/async.dart';
 import 'package:path/path.dart' as p;
 import 'package:stack_trace/stack_trace.dart';
 
 import '../backend/invoker.dart';
 import '../backend/metadata.dart';
 import '../backend/test_platform.dart';
 import '../util/dart.dart' as dart;
 import '../util/io.dart';
 import '../util/remote_exception.dart';
 import '../utils.dart';
+import 'browser/server.dart';
 import 'configuration.dart';
-import 'browser/server.dart';
+import 'hack_load_vm_file_hook.dart';
 import 'load_exception.dart';
 import 'load_suite.dart';
 import 'parse_metadata.dart';
 import 'runner_suite.dart';
 import 'vm/environment.dart';
 import 'vm/isolate_test.dart';
 
 /// A class for finding test files and loading them into a runnable form.
 class Loader {
   /// The test runner configuration.
@@ skipping 102 matching lines @@
   ///
   /// [metadata] is the suite-level metadata for the test.
   Future<RunnerSuite> _loadBrowserFile(String path, TestPlatform platform,
         Metadata metadata) async =>
       (await _browserServer).loadSuite(path, platform, metadata);
 
   /// Load the test suite at [path] in VM isolate.
   ///
   /// [metadata] is the suite-level metadata for the test.
   Future<RunnerSuite> _loadVmFile(String path, Metadata metadata) async {
+    if (loadVMFileHook != null) {
+      var suite = await loadVMFileHook(path, metadata, _config);
+      _suites.add(suite);
+      return suite;
+    }
+
     var receivePort = new ReceivePort();
 
     var isolate;
     try {
       if (_config.pubServeUrl != null) {
         var url = _config.pubServeUrl.resolveUri(
             p.toUri(p.relative(path, from: 'test') + '.vm_test.dart'));
 
         try {
           isolate = await Isolate.spawnUri(url, [], {
@@ skipping 81 matching lines @@
   Future close() {
     return _closeMemo.runOnce(() async {
       await Future.wait(_suites.map((suite) => suite.close()));
       _suites.clear();
 
       if (!_browserServerMemo.hasRun) return;
       await (await _browserServer).close();
     });
   }
 }