Add a platform plugin infrastructure.

lib/src/runner/environment.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.
 
 import 'package:async/async.dart';
 
 /// The abstract class of environments in which test suites are
 /// loaded—specifically, browsers and the Dart VM.
 abstract class Environment {
+  /// Whether this environment supports interactive debugging.
+  bool get supportsDebugging;
+
   /// The URL of the Dart VM Observatory for this environment, or `null` if this
   /// environment doesn't run the Dart VM or the URL couldn't be detected.
   Uri get observatoryUrl;
 
   /// The URL of the remote debugger for this environment, or `null` if it isn't
   /// enabled.
   Uri get remoteDebuggerUrl;
 
   /// Displays information indicating that the test runner is paused.
   ///
   /// The returned operation will complete when the user takes action within the
   /// environment that should unpause the runner. If the runner is unpaused
   /// elsewhere, the operation should be canceled.
   CancelableOperation displayPause();
 }