DAS plugins: minor adjustments regarding the PluginIsolateChannel

It looks like PluginCommunicationChannel.listen cannot do anything
with `onError`, so don't pass anything as if it did.

See ReceivePort.listen docs as well: https://api.dart.dev/dev/latest/dart-isolate/ReceivePort/listen.html

Change-Id: Iaaf1169a1f05774f63bdcea4c91a82dfb9e47227
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396526
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/analysis_server_plugin/lib/src/plugin_server.dart

@@ -180,8 +180,6 @@ class PluginServer {
   void start(PluginCommunicationChannel channel) {
     _channel = channel;
     _channel.listen(_handleRequestZoned,
-        // TODO(srawlins): Implement.
-        onError: (_) {},
         // TODO(srawlins): Implement.
         onDone: () {});
   }

pkg/analyzer_plugin/lib/channel/channel.dart

@@ -2,6 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+/// @docImport 'package:analyzer_plugin/plugin/plugin.dart';
+library;
+
 import 'package:analyzer_plugin/protocol/protocol.dart';
 
 /// A communication channel that allows a [ServerPlugin] to receive [Request]s

pkg/analyzer_plugin/lib/src/channel/isolate_channel.dart

@@ -102,6 +102,10 @@ class PluginIsolateChannel implements PluginCommunicationChannel {
     if (_subscription != null) {
       throw StateError('Only one listener is allowed per channel');
     }
+    // Note: The documentation for [ReceivePort.listen] indicates that
+    // `onError` and `cancelOnError` are always ignored, "since a ReceivePort
+    // will never receive an error." It is potentially confusing to pass them
+    // here, as if they had an effect. Consider removing.
     _subscription = _receivePort.listen(onData,
         onError: onError, onDone: onDone, cancelOnError: false);
   }