Report hint on unmatched show and hide combinators.

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

 // 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:html library.
 
 /**
  * HTML elements and other resources for web-based applications that need to
@@ skipping 16 matching lines @@
  * and run its built-in examples.
  *
  * * For even more examples, see
  * [Dart HTML5 Samples](https://github.com/dart-lang/dart-html5-samples)
  * on Github.
  */
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
-import 'dart:_internal' hide Symbol, deprecated;
+import 'dart:_internal' hide Symbol;
 import 'dart:html_common';
 import 'dart:indexed_db';
 import 'dart:isolate';
 import "dart:convert";
 import 'dart:math';
 import 'dart:_native_typed_data';
 import 'dart:typed_data';
 // Not actually used, but imported since dart:html can generate these objects.
 import 'dart:svg' as svg;
 import 'dart:svg' show Matrix;
 import 'dart:svg' show SvgSvgElement;
 import 'dart:web_audio' as web_audio;
 import 'dart:web_gl' as gl;
 import 'dart:web_gl' show RenderingContext;
 import 'dart:web_sql';
 import 'dart:_js_helper' show
     convertDartClosureToJS, Creates, JavaScriptIndexingBehavior,
-    JSName, Native, Null, Returns, Inline, ForceInline,
+    JSName, Native, Returns, ForceInline,
     findDispatchTagForInterceptorClass, setNativeSubclassDispatchRecord,
     makeLeafDispatchRecord;
 import 'dart:_interceptors' show
     Interceptor, JSExtendableArray, JSUInt31,
     findInterceptorConstructorForType,
     findConstructorForNativeSubclassType,
     getNativeInterceptor,
     setDispatchProperty;
 import 'dart:_isolate_helper' show IsolateNatives;
 import 'dart:_foreign_helper' show JS, JS_INTERCEPTOR_CONSTANT, JS_CONST;
@@ skipping 76 matching lines @@
  * Spawn a DOM isolate using the given URI in the same window.
  * This isolate is not concurrent.  It runs on the browser thread
  * with full access to the DOM.
  * Note: this API is still evolving and may move to dart:isolate.
  */
 @Experimental()
 Future<Isolate> spawnDomUri(Uri uri, List<String> args, message) {
   // TODO(17738): Implement this.
   throw new UnimplementedError();
 }