Add a platform plugin infrastructure.

test/runner/load_suite_test.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.
 
 @TestOn("vm")
 
 import 'dart:async';
 
 import 'package:test/src/backend/group.dart';
 import 'package:test/src/backend/state.dart';
 import 'package:test/src/backend/test_platform.dart';
 import 'package:test/src/runner/load_exception.dart';
 import 'package:test/src/runner/load_suite.dart';
+import 'package:test/src/runner/plugin/environment.dart';
 import 'package:test/src/runner/runner_suite.dart';
-import 'package:test/src/runner/vm/environment.dart';
 import 'package:test/test.dart';
 
 import '../utils.dart';
 
 void main() {
   var innerSuite;
   setUp(() {
-    innerSuite = new RunnerSuite(const VMEnvironment(), new Group.root([]));
+    innerSuite = new RunnerSuite(const PluginEnvironment(), new Group.root([]));
   });
 
   test("running a load test causes LoadSuite.suite to emit a suite", () async {
     var suite = new LoadSuite("name", () => new Future.value(innerSuite));
     expect(suite.group.entries, hasLength(1));
 
     expect(suite.suite, completion(equals(innerSuite)));
     var liveTest = await suite.group.entries.single.load(suite);
     await liveTest.run();
     expectTestPassed(liveTest);
@@ skipping 84 matching lines @@
       var newSuite = suite.changeSuite((suite) => suite);
       expect(newSuite.platform, equals(TestPlatform.vm));
       expect(newSuite.group.entries.single.name, equals(suite.group.entries.single.name));
     });
 
     test("changes the inner suite", () async {
       var suite = new LoadSuite("name", () => innerSuite);
       expect(suite.group.entries, hasLength(1));
 
       var newInnerSuite = new RunnerSuite(
-          const VMEnvironment(), new Group.root([]));
+          const PluginEnvironment(), new Group.root([]));
       var newSuite = suite.changeSuite((suite) => newInnerSuite);
       expect(newSuite.suite, completion(equals(newInnerSuite)));
 
       var liveTest = await suite.group.entries.single.load(suite);
       await liveTest.run();
       expectTestPassed(liveTest);
     });
 
     test("doesn't run change() if the suite is null", () async {
       var suite = new LoadSuite("name", () => null);
@@ skipping 17 matching lines @@
     });
 
     test("forwards errors to the future", () {
       var suite = new LoadSuite("name", () => throw "error");
       expect(suite.group.entries, hasLength(1));
 
       expect(suite.getSuite(), throwsA("error"));
     });
   });
 }