DAS plugins: onError must accept one parameter

It's awkward that PluginCommunicationChannel.listen accepts a
`Function?` for the `onError` parameter, but it _must_ be a
`void Function(Object)` or a `void Function(Object, StackTrace)`. I
wish we had static analysis for this...

Change-Id: Id91a8172c7ef20e35bb8ff21736649d1501aff42
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395082
Commit-Queue: Samuel Rawlins <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: srawlins

pkg/analysis_server_plugin/lib/src/plugin_server.dart

@@ -166,7 +166,7 @@ class PluginServer {
     _channel = channel;
     _channel.listen(_handleRequestZoned,
         // TODO(srawlins): Implement.
-        onError: () {},
+        onError: (_) {},
         // TODO(srawlins): Implement.
         onDone: () {});
   }