Created a skeleton for the backend of Dart Debug Extension.

dwds/lib/dwds.dart

@@ -12,10 +12,10 @@ import 'package:webkit_inspection_protocol/webkit_inspection_protocol.dart';
 
 import 'src/connections/app_connection.dart';
 import 'src/connections/debug_connection.dart';
-import 'src/devtools.dart';
 import 'src/handlers/asset_handler.dart';
 import 'src/handlers/dev_handler.dart';
 import 'src/handlers/injected_handler.dart';
+import 'src/servers/devtools.dart';
 
 export 'src/connections/app_connection.dart' show AppConnection;
 export 'src/connections/debug_connection.dart' show DebugConnection;

dwds/lib/src/handlers/dev_handler.dart

@@ -18,9 +18,9 @@ import '../../data/devtools_request.dart';
 import '../../data/isolate_events.dart';
 import '../../data/serializers.dart';
 import '../connections/app_connection.dart';
-import '../devtools.dart';
 import '../dwds_vm_client.dart';
 import '../handlers/asset_handler.dart';
+import '../servers/devtools.dart';
 import '../services/app_debug_services.dart';
 import '../services/debug_service.dart';
 

dwds/lib/src/servers/extension_backend.dart

@@ -0,0 +1,45 @@
+// Copyright (c) 2019, 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:io';
+
+import 'package:http_multi_server/http_multi_server.dart';
+import 'package:shelf/shelf_io.dart';
+import 'package:sse/server/sse_handler.dart';
+
+SseHandler _sseHandler = SseHandler(Uri.parse('/test'));
+
+/// A backend for the Dart Debug Extension which sets up an SSE handler.
+///
+/// The SSE handler simply sends a simple message and prints the response when
+/// a client connects.
+class ExtensionBackend {
+  final String hostname;
+  final int port;
+  final HttpServer _server;
+
+  ExtensionBackend._(this.hostname, this.port, this._server) {
+    _listen();
+  }
+
+  // Starts the backend on an open port.
+  static Future<ExtensionBackend> start() async {
+    var server = await HttpMultiServer.bind('localhost', 0);
+    serveRequests(server, _sseHandler.handler);
+    return ExtensionBackend._(server.address.host, server.port, server);
+  }
+
+  Future<void> close() async {
+    await _server.close();
+  }
+
+  void _listen() async {
+    var connections = _sseHandler.connections;
+    while (await connections.hasNext) {
+      var connection = await connections.next;
+      connection.sink.add('foo');
+      connection.stream.listen(print);
+    }
+  }
+}