small fix to make analyzer happy

R=nbosch@google.com

Review URL: https://codereview.chromium.org//2344893007 .
diff --git a/lib/shelf_io.dart b/lib/shelf_io.dart
index 4991089..58fbd74 100644
--- a/lib/shelf_io.dart
+++ b/lib/shelf_io.dart
@@ -116,8 +116,8 @@
     headers[k] = v.join(',');
   });
 
-  onHijack(callback) {
-    return request.response
+  void onHijack(callback) {
+    request.response
         .detachSocket(writeHeaders: false)
         .then((socket) => callback(socket, socket));
   }