Report hint on unmatched show and hide combinators.

sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart

 /**
  * A client-side key-value store with support for indexes.
  *
  * Many browsers support IndexedDB—a web standard for
  * an indexed database.
  * By storing data on the client in an IndexedDB,
  * a web app gets some advantages, such as faster performance and persistence.
  * To find out which browsers support IndexedDB,
  * refer to [Can I Use?](http://caniuse.com/#feat=indexeddb)
  *
@@ skipping 59 matching lines @@
  * [IndexedDB reference](http://docs.webplatform.org/wiki/apis/indexeddb)
  * provides wiki-style docs about indexedDB
  */
 library dart.dom.indexed_db;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:_native_typed_data';
 import 'dart:typed_data';
-import 'dart:_js_helper' show Creates, Returns, JSName, Native, Null;
+import 'dart:_js_helper' show Creates, Returns, JSName, Native;
 import 'dart:_foreign_helper' show JS;
 import 'dart:_interceptors' show Interceptor, JSExtendableArray;
 // 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.
 
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:svg library.
 
@@ skipping 1355 matching lines @@
   @Creates('int|String|Null')
   @Returns('int|String|Null')
   final int newVersion;
 
   @DomName('IDBVersionChangeEvent.oldVersion')
   @DocsEditable()
   @Creates('int|String|Null')
   @Returns('int|String|Null')
   final int oldVersion;
 }