Add a platform plugin infrastructure.

lib/test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 
 import 'src/backend/declarer.dart';
 import 'src/backend/test_platform.dart';
 import 'src/frontend/timeout.dart';
 import 'src/runner/engine.dart';
+import 'src/runner/plugin/environment.dart';
 import 'src/runner/reporter/expanded.dart';
 import 'src/runner/runner_suite.dart';
-import 'src/runner/vm/environment.dart';
 import 'src/utils.dart';
 
 export 'package:matcher/matcher.dart';
 
 export 'src/frontend/expect.dart';
 export 'src/frontend/expect_async.dart';
 export 'src/frontend/future_matchers.dart';
 export 'src/frontend/on_platform.dart';
 export 'src/frontend/prints_matcher.dart';
 export 'src/frontend/skip.dart';
@@ skipping 18 matching lines @@
   if (declarer != null) return declarer;
   if (_globalDeclarer != null) return _globalDeclarer;
 
   // Since there's no Zone-scoped declarer, the test file is being run directly.
   // In order to run the tests, we set up our own Declarer via
   // [_globalDeclarer], and schedule a microtask to run the tests once they're
   // finished being defined.
   _globalDeclarer = new Declarer();
   scheduleMicrotask(() async {
     var suite = new RunnerSuite(
-        // TODO(nweiz): Use a different environment if VM environment starts
-        // import dart:io before cross-platform libraries exist.
-        const VMEnvironment(),
+        const PluginEnvironment(),
         _globalDeclarer.build(),
         path: p.prettyUri(Uri.base),
         platform: TestPlatform.vm,
         os: currentOSGuess);
 
     var engine = new Engine();
     engine.suiteSink.add(suite);
     engine.suiteSink.close();
     ExpandedReporter.watch(engine,
         color: true, printPath: false, printPlatform: false);
@@ skipping 169 matching lines @@
 /// prefer [tearDown], and only use [tearDOwnAll] if the callback is
 /// prohibitively slow.
 void tearDownAll(callback()) => _declarer.tearDownAll(callback);
 
 /// Registers an exception that was caught for the current test.
 void registerException(error, [StackTrace stackTrace]) {
   // This will usually forward directly to [Invoker.current.handleError], but
   // going through the zone API allows other zones to consistently see errors.
   Zone.current.handleUncaughtError(error, stackTrace);
 }