Report hint on unmatched show and hide combinators.

pkg/compiler/lib/src/deferred_load.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 deferred_load;
 
 import 'common/backend_api.dart' show
     Backend;
 import 'common/tasks.dart' show
     CompilerTask;
@@ skipping 16 matching lines @@
     Element,
     ElementKind,
     Elements,
     ExportElement,
     FunctionElement,
     ImportElement,
     LibraryElement,
     MetadataAnnotation,
     PrefixElement,
     ScopeContainerElement,
-    TypedefElement,
-    VoidElement;
+    TypedefElement;
 import 'js_backend/js_backend.dart' show
     JavaScriptBackend;
 import 'resolution/resolution.dart' show
     AnalyzableElementX;
 import 'resolution/tree_elements.dart' show
     TreeElements;
 import 'tree/tree.dart' as ast;
 import 'tree/tree.dart' show
     Import,
     LibraryTag,
@@ skipping 888 matching lines @@
     return result;
   }
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 }