Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/analyzer/element_store.dart

 // Copyright (c) 2016, 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 fasta.analyzer.element_store;
 
 import 'package:kernel/analyzer/loader.dart' show
     ReferenceLevelLoader;
 
 import 'package:kernel/ast.dart';
@@ skipping 101 matching lines @@
         ? cls.cls.constructors.first
         : cls.cls.constructors.firstWhere(
             (Constructor c) => c.name.name == constructorName);
     return constructor;
   }
 
   Library getLibraryReference(LibraryElement element) {
     return internalError("not supported.");
   }
 
-  Class getClassReference(KernelClassElement cls) => cls.builder.cls;
+  Class getClassReference(covariant KernelClassElement cls) => cls.builder.cls;
 
   Member getMemberReference(Element element) {
     if (element is KernelFunctionElement) {
       return element.procedure;
     } else {
       return internalError("getMemberReference(${element.runtimeType})");
     }
   }
 
   Class getRootClassReference() => internalError("not supported.");
@@ skipping 153 matching lines @@
   }
 }
 
 class KernelInterfaceType extends MockInterfaceType {
   final KernelClassElement element;
 
   KernelInterfaceType(this.element);
 
   List<analyzer.DartType> get typeArguments => const <analyzer.DartType>[];
 }