Add a platform plugin infrastructure.

lib/src/runner/browser/browser_manager.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 'dart:async';
 import 'dart:convert';
 
 import 'package:async/async.dart';
 import 'package:http_parser/http_parser.dart';
 import 'package:pool/pool.dart';
@@ skipping 223 matching lines @@
   });
   final _closeMemoizer = new AsyncMemoizer();
 }
 
 /// An implementation of [Environment] for the browser.
 ///
 /// All methods forward directly to [BrowserManager].
 class _BrowserEnvironment implements Environment {
   final BrowserManager _manager;
 
+  final supportsDebugging = true;
+
   final Uri observatoryUrl;
 
   final Uri remoteDebuggerUrl;
 
   _BrowserEnvironment(this._manager, this.observatoryUrl,
       this.remoteDebuggerUrl);
 
   CancelableOperation displayPause() => _manager._displayPause();
 }