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 118 matching lines @@
   AnalyzerLocalVariableElemment(this.variable)
       : super(ElementKind.LOCAL_VARIABLE);
 
   String get name => variable.name.name;
 
   bool get isFinal => false; // TODO(ahe): implement this.
 
   bool get isConst => false; // TODO(ahe): implement this.
 
   analyzer.VariableDeclaration get target => variable;
+
+  get type => null;
+
+  get constantValue => internalError("not supported.");
+
+  computeConstantValue() => internalError("not supported.");
 }
 
 /// Both an [Element] and [Builder] to using memory to store local elements in
 /// [ElementStore].
 class AnalyzerParameterElement extends MockParameterElement with Builder {
   final analyzer.FormalParameter parameter;
 
   AnalyzerParameterElement(this.parameter);
 
   String get name => parameter.identifier.name;
@@ skipping 15 matching lines @@
   }
 }
 
 class KernelInterfaceType extends MockInterfaceType {
   final KernelClassElement element;
 
   KernelInterfaceType(this.element);
 
   List<analyzer.DartType> get typeArguments => const <analyzer.DartType>[];
 }