Fix hints reported by analyzer.

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

 // Copyright (c) 2012, 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 js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 880 matching lines @@
       return elements
           .where((element) => classesMixedIntoInterceptedClasses
               .contains(element.enclosingClass))
           .toSet();
     });
 
     if (elements == null) return false;
     if (elements.isEmpty) return false;
     return elements.any((element) {
       return selector.applies(element) &&
-          (mask == null || mask.canHit(element, selector, _closedWorld));
+          (mask == null ||
+              mask.canHit(element as MemberElement, selector, _closedWorld));
     });
   }
 
   /// True if the given class is an internal class used for type inference
   /// and never exists at runtime.
   bool isCompileTimeOnlyClass(ClassElement class_) {
     return class_ == helpers.jsPositiveIntClass ||
         class_ == helpers.jsUInt32Class ||
         class_ == helpers.jsUInt31Class ||
         class_ == helpers.jsFixedArrayClass ||
@@ skipping 2374 matching lines @@
   @override
   bool isNativeClass(ClassElement element) {
     return helpers.backend.isNative(element);
   }
 
   @override
   bool isNativeMember(MemberElement element) {
     return helpers.backend.isNative(element);
   }
 }