First batch of changes to make analyzer package strong mode error free

pkg/analyzer/lib/src/plugin/options_plugin.dart

Index: pkg/analyzer/lib/src/plugin/options_plugin.dart
diff --git a/pkg/analyzer/lib/src/plugin/options_plugin.dart b/pkg/analyzer/lib/src/plugin/options_plugin.dart
index 6fba6493b8bf288be683f9c34845c03cf682851b..9a4fc01386ec592e2e39d23134ac6aa3004a2134 100644
--- a/pkg/analyzer/lib/src/plugin/options_plugin.dart
+++ b/pkg/analyzer/lib/src/plugin/options_plugin.dart
@@ -34,11 +34,13 @@ class OptionsPlugin implements Plugin {
 
   /// All contributed options processors.
   List<OptionsProcessor> get optionsProcessors =>
-      optionsProcessorExtensionPoint?.extensions ?? const [];
+      optionsProcessorExtensionPoint?.extensions as List<OptionsProcessor> ??
+      const <OptionsProcessor>[];
 
   /// All contributed options validators.
   List<OptionsValidator> get optionsValidators =>
-      optionsValidatorExtensionPoint?.extensions ?? const [];
+      optionsValidatorExtensionPoint?.extensions as List<OptionsValidator> ??
+      const <OptionsValidator>[];
 
   @override
   String get uniqueIdentifier => UNIQUE_IDENTIFIER;