Fix declaration of _MixinFieldNamingScope.names.

pkg/compiler/lib/src/js_backend/minify_namer.dart

 // Copyright (c) 2011, 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.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class MinifyNamer extends Namer {
@@ skipping 394 matching lines @@
  * all applications of a mixin. To achieve this, we use global naming for
  * mixins from the same name pool as fields and add a `$` at the end to ensure
  * they do not collide with normal field names. The `$` sign is typically used
  * as a separator between method names and argument counts and does not appear
  * in generated names themselves.
  */
 class _MixinFieldNamingScope extends _FieldNamingScope {
   int get _localFieldNameCounter => registry.globalCount;
   void set _localFieldNameCounter(int val) { registry.globalCount = val; }
 
-  Map<Element, String> get names => registry.globalNames;
+  Map<Entity, String> get names => registry.globalNames;
 
   _MixinFieldNamingScope.mixin(ClassElement cls, _FieldNamingRegistry registry)
     : super.rootScope(cls, registry);
 
   _MixinFieldNamingScope.mixedIn(MixinApplicationElement container,
       _FieldNamingScope superScope, _FieldNamingRegistry registry)
     : super.inherit(container, superScope, registry);
 
   String _nextName() {
     var proposed = super._nextName();
@@ skipping 11 matching lines @@
   _BoxFieldNamingScope(Local box, _FieldNamingRegistry registry) :
     super.rootScope(box, registry);
 
   bool containsField(_) => true;
 
   String operator[](Element field) {
     if (!names.containsKey(field)) add(field);
     return names[field];
   }
 }