analyzer: Make AnalysisOptions pluginConfigurations final.

Change-Id: I233c844f1e3aae662fc8755e700bc69049145aa3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395340
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/analyzer/lib/src/dart/analysis/analysis_options.dart

@@ -326,7 +326,7 @@ class AnalysisOptionsImpl implements AnalysisOptions {
   final List<String> enabledLegacyPluginNames;
 
   @override
-  List<PluginConfiguration> pluginConfigurations = [];
+  final List<PluginConfiguration> pluginConfigurations;
 
   @override
   final List<ErrorProcessor> errorProcessors;