Make analyzer DDC warning clean.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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 engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 5138 matching lines @@
 
   /**
    * Initialize a newly created visitor to build implicit constructors.
    *
    * The visit methods will pass closures to [_callback] to indicate what
    * computation needs to be performed, and its dependency order.
    */
   ImplicitConstructorBuilder(this.errorListener, this._callback);
 
   @override
-  void visitClassElement(ClassElementImpl classElement) {
-    classElement.mixinErrorsReported = false;
+  void visitClassElement(ClassElement classElement) {
+    (classElement as ClassElementImpl).mixinErrorsReported = false;
     if (classElement.isTypedef) {
       _visitClassTypeAlias(classElement);
     } else {
       _visitClassDeclaration(classElement);
     }
   }
 
   @override
   void visitCompilationUnitElement(CompilationUnitElement element) {
     element.types.forEach(visitClassElement);
@@ skipping 10299 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }