DAS plugins: fix bad parsing of unsupported requests

I noticed only recently that every time I create a new source file in
a package which is being analyzed with analyzer plugins, I get an
"exception," a toast popup in the IDE. It turns out I goofed in
https://github.com/dart-lang/sdk/commit/da1d59c4e6cdb65b48db8ff182cf77f377ca8d1b
and totally busted the switch statement. The request types above
ANALYSIS_REQUEST_HANDLE_WATCH_EVENTS still flow into it's body. Ouch.

Change-Id: I3ba9fecfb9c3a1e8585e472b09fa0b2f78ba18df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449145
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server_plugin/lib/src/plugin_server.dart

@@ -542,7 +542,6 @@ class PluginServer {
   Future<Response?> _getResponse(Request request, int requestTime) async {
     ResponseResult? result;
     switch (request.method) {
-      case protocol.ANALYSIS_REQUEST_GET_NAVIGATION:
       case protocol.ANALYSIS_REQUEST_HANDLE_WATCH_EVENTS:
         var params = protocol.AnalysisHandleWatchEventsParams.fromRequest(
           request,
@@ -556,28 +555,21 @@ class PluginServer {
         result = await _handleAnalysisSetContextRoots(params);
 
       case protocol.ANALYSIS_REQUEST_SET_PRIORITY_FILES:
-      case protocol.ANALYSIS_REQUEST_SET_SUBSCRIPTIONS:
+        // TODO(srawlins): Support!
+        result = null;
+
       case protocol.ANALYSIS_REQUEST_UPDATE_CONTENT:
         var params = protocol.AnalysisUpdateContentParams.fromRequest(request);
         result = await _handleAnalysisUpdateContent(params);
 
-      case protocol.COMPLETION_REQUEST_GET_SUGGESTIONS:
-        result = null;
-
       case protocol.EDIT_REQUEST_GET_ASSISTS:
         var params = protocol.EditGetAssistsParams.fromRequest(request);
         result = await handleEditGetAssists(params);
 
-      case protocol.EDIT_REQUEST_GET_AVAILABLE_REFACTORINGS:
-        result = null;
-
       case protocol.EDIT_REQUEST_GET_FIXES:
         var params = protocol.EditGetFixesParams.fromRequest(request);
         result = await handleEditGetFixes(params);
 
-      case protocol.EDIT_REQUEST_GET_REFACTORING:
-        result = null;
-
       case protocol.PLUGIN_REQUEST_DETAILS:
         var details = <protocol.PluginDetails>[];
         for (var pluginRegistry in _registries) {
@@ -612,6 +604,10 @@ class PluginServer {
       case protocol.PLUGIN_REQUEST_VERSION_CHECK:
         var params = protocol.PluginVersionCheckParams.fromRequest(request);
         result = await handlePluginVersionCheck(params);
+
+      default:
+        // Anything else is unsupported.
+        result = null;
     }
     if (result == null) {
       return Response(