Mark analysis_server as being strong-mode clean

pkg/analysis_server/lib/src/channel/web_socket_channel.dart

Index: pkg/analysis_server/lib/src/channel/web_socket_channel.dart
diff --git a/pkg/analysis_server/lib/src/channel/web_socket_channel.dart b/pkg/analysis_server/lib/src/channel/web_socket_channel.dart
index 0368eb64a6e5726a41a4d359dabafc750289eccf..282b1f965fecb814f7e2b012abaaa169a091fdef 100644
--- a/pkg/analysis_server/lib/src/channel/web_socket_channel.dart
+++ b/pkg/analysis_server/lib/src/channel/web_socket_channel.dart
@@ -55,10 +55,11 @@ class WebSocketClientChannel implements ClientCommunicationChannel {
   }
 
   @override
-  Future<Response> sendRequest(Request request) {
+  Future<Response> sendRequest(Request request) async {
     String id = request.id;
     socket.add(JSON.encode(request.toJson()));
-    return responseStream.firstWhere((Response response) => response.id == id);
+    return await responseStream
+        .firstWhere((Response response) => response.id == id);
   }
 }