[ package:dds ] Fix TypeError when attempting to debug DevTools requests

TEST=Manual

Change-Id: I043d52fe8863e2ea159300990db48dd05b7bc199
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280051
Reviewed-by: Dan Chevalier <danchevalier@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/dds/lib/src/devtools/client.dart

@@ -186,7 +186,7 @@ class DevToolsClient {
         print('DevTools SSE request: $e');
         return e;
       });
-      sink = LoggingMiddlewareSink(sink);
+      sink = LoggingMiddlewareSink<String>(sink);
     }
 
     _devToolsPeer = json_rpc.Peer(