Report hint on unmatched show and hide combinators.

pkg/compiler/lib/src/compiler.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 dart2js.compiler_base;
 
 import 'dart:async' show
     EventSink,
     Future;
 
@@ skipping 51 matching lines @@
 import 'dump_info.dart' show
     DumpInfoTask;
 import 'elements/elements.dart';
 import 'elements/modelx.dart' show
     ErroneousElementX,
     ClassElementX,
     CompilationUnitElementX,
     DeferredLoaderGetterElementX,
     MethodElementX,
     LibraryElementX,
-    PrefixElementX,
-    VoidElementX;
+    PrefixElementX;
 import 'enqueue.dart' show
     CodegenEnqueuer,
     Enqueuer,
     EnqueueTask,
     ResolutionEnqueuer,
     QueueFilter,
     WorldImpact;
 import 'io/source_information.dart' show
     SourceInformation;
 import 'js_backend/js_backend.dart' as js_backend show
     JavaScriptBackend;
 import 'library_loader.dart' show
     LibraryLoader,
     LibraryLoaderTask,
     LoadedLibraries;
 import 'mirrors_used.dart' show
     MirrorUsageAnalyzerTask;
 import 'null_compiler_output.dart' show
     NullCompilerOutput,
     NullSink;
 import 'parser/diet_parser_task.dart' show
     DietParserTask;
 import 'parser/parser_task.dart' show
-    DietParserTask,
     ParserTask;
 import 'patch_parser.dart' show
     PatchParserTask;
 import 'resolution/registry.dart' show
     ResolutionRegistry;
 import 'resolution/resolution.dart' show
     ResolverTask;
 import 'resolution/tree_elements.dart' show
     TreeElementMapping;
 import 'scanner/scanner_task.dart' show
@@ skipping 1722 matching lines @@
 
   @override
   InterfaceType streamType([DartType elementType]) {
     InterfaceType type = streamClass.computeType(compiler);
     if (elementType == null) {
       return streamClass.rawType;
     }
     return type.createInstantiation([elementType]);
   }
 }