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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library analyzer.src.plugin.options_plugin;
 
 import 'package:analyzer/plugin/options.dart';
 import 'package:analyzer/plugin/task.dart';
 import 'package:analyzer/src/task/options.dart';
 import 'package:plugin/plugin.dart';
@@ skipping 16 matching lines @@
   /// The extension point that allows plugins to register new options
   /// processors.
   ExtensionPoint optionsProcessorExtensionPoint;
 
   /// The extension point that allows plugins to register new options
   /// validators.
   ExtensionPoint optionsValidatorExtensionPoint;
 
   /// 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;
 
   @override
   void registerExtensionPoints(RegisterExtensionPoint registerExtensionPoint) {
     optionsProcessorExtensionPoint = registerExtensionPoint(
         OPTIONS_PROCESSOR_EXTENSION_POINT, _validateOptionsProcessorExtension);
     optionsValidatorExtensionPoint = registerExtensionPoint(
         OPTIONS_VALIDATOR_EXTENSION_POINT, _validateOptionsValidatorExtension);
@@ skipping 20 matching lines @@
 
   /// Validate the given extension by throwing an [ExtensionError] if it is not
   /// a valid options validator.
   void _validateOptionsValidatorExtension(Object extension) {
     if (extension is! OptionsValidator) {
       String id = optionsValidatorExtensionPoint.uniqueIdentifier;
       throw new ExtensionError('Extensions to $id must be an OptionsValidator');
     }
   }
 }