Add a platform plugin infrastructure.

lib/src/runner/debugger.dart

 // Copyright (c) 2016, 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:async/async.dart';
 
-import '../backend/test_platform.dart';
 import '../util/io.dart';
 import '../utils.dart';
 import 'configuration.dart';
 import 'console.dart';
 import 'engine.dart';
 import 'load_suite.dart';
 import 'reporter.dart';
 import 'runner_suite.dart';
 
 /// Runs [loadSuite] in debugging mode.
@@ skipping 83 matching lines @@
       await _engine.onIdle.first;
     } finally {
       close();
     }
   }
 
   /// Prints URLs for the [_suite]'s debugger and waits for the user to tell the
   /// suite to run.
   Future _pause() async {
     if (_suite.platform == null) return;
-    if (_suite.platform == TestPlatform.vm) return;
+    if (!_suite.environment.supportsDebugging) return;
 
     try {
       _reporter.pause();
 
       var bold = _config.color ? '\u001b[1m' : '';
       var yellow = _config.color ? '\u001b[33m' : '';
       var noColor = _config.color ? '\u001b[0m' : '';
       print('');
 
       if (_suite.platform.isDartVM) {
@@ skipping 64 matching lines @@
         'Will restart "${liveTest.test.name}" once it finishes running.'));
   }
 
   /// Closes the debugger and releases its resources.
   void close() {
     _closed = true;
     _onDebuggingSubscription.cancel();
     _console.stop();
   }
 }