Add a platform plugin infrastructure.

lib/src/runner/load_suite.dart

Index: lib/src/runner/load_suite.dart
diff --git a/lib/src/runner/load_suite.dart b/lib/src/runner/load_suite.dart
index 7f0778736e239466757e261c3070c4e95a8122ea..08cbda308964aeb477ac0eef4f55dcdd84a5fc67 100644
--- a/lib/src/runner/load_suite.dart
+++ b/lib/src/runner/load_suite.dart
@@ -15,8 +15,8 @@ import '../backend/test.dart';
 import '../backend/test_platform.dart';
 import '../utils.dart';
 import 'load_exception.dart';
+import 'plugin/environment.dart';
 import 'runner_suite.dart';
-import 'vm/environment.dart';
 
 /// A [Suite] emitted by a [Loader] that provides a test-like interface for
 /// loading a test file.
@@ -33,7 +33,7 @@ import 'vm/environment.dart';
 /// suite itself is returned by [suite] once it's avaialble, but any errors or
 /// prints will be emitted through the running [LiveTest].
 class LoadSuite extends Suite implements RunnerSuite {
-  final environment = const VMEnvironment();
+  final environment = const PluginEnvironment();
   final isDebugging = false;
   final onDebugging = new StreamController<bool>().stream;