Bump sse to v3.1.1 and add server keepalive

This allows the server to accept client reconnections and present them transparently to the consuming code (see https://github.com/dart-lang/sse/issues/18).

dwds/debug_extension/pubspec.yaml

@@ -14,7 +14,7 @@ dependencies:
   js: ^0.6.1+1
   pedantic: ^1.5.0
   pub_semver: ^1.3.2
-  sse: ^3.0.0
+  sse: ^3.1.1
 
 dev_dependencies:
   webdev: ^2.0.0

dwds/lib/src/handlers/dev_handler.dart

@@ -36,7 +36,8 @@ import '../services/debug_service.dart';
 /// opening DevTools.
 class DevHandler {
   StreamSubscription _sub;
-  final SseHandler _sseHandler = SseHandler(Uri.parse(r'/$sseHandler'));
+  final SseHandler _sseHandler = SseHandler(Uri.parse(r'/$sseHandler'),
+      keepAlive: const Duration(seconds: 30));
   final _injectedConnections = Set<SseConnection>();
   final DevTools _devTools;
   final AssetReader _assetReader;

dwds/lib/src/servers/extension_backend.dart

@@ -11,7 +11,8 @@ import 'package:http_multi_server/http_multi_server.dart';
 import 'package:shelf/shelf_io.dart';
 import 'package:sse/server/sse_handler.dart';
 
-final _sseHandler = SseHandler(Uri.parse('/\$debug'));
+final _sseHandler =
+    SseHandler(Uri.parse('/\$debug'), keepAlive: const Duration(seconds: 30));
 
 /// A backend for the Dart Debug Extension.
 ///

dwds/pubspec.yaml

@@ -30,7 +30,7 @@ dependencies:
   shelf_static: ^0.2.8
   shelf_web_socket: ^0.2.0
   source_maps: ^0.10.0
-  sse: ^3.0.0
+  sse: ^3.1.1
   vm_service: 1.2.0
   web_socket_channel: ^1.0.0
   webkit_inspection_protocol: '>=0.4.0 <0.6.0'

webdev/pubspec.yaml

@@ -29,7 +29,7 @@ dependencies:
   shelf: ^0.7.4
   shelf_proxy: ^0.1.0+5
   shelf_static: ^0.2.8
-  sse: ^3.0.0
+  sse: ^3.1.1
   vm_service: ^1.1.0
   webkit_inspection_protocol: '>=0.4.0 <0.6.0'
   yaml: ^2.1.13