Diagnose missing @MirrorsUsed.

dart/sdk/lib/_internal/compiler/implementation/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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1308 matching lines @@
           {'count': mirrorCount,
            'total': totalMethodCount,
            'percentage': percentage.round()});
       for (LibraryElement library in compiler.libraries.values) {
         if (library.isInternalLibrary) continue;
         for (LibraryTag tag in library.tags) {
           Import importTag = tag.asImport();
           if (importTag == null) continue;
           LibraryElement importedLibrary = library.getLibraryFromTag(tag);
           if (importedLibrary != compiler.mirrorsLibrary) continue;
+          MessageKind kind =
+              compiler.mirrorUsageAnalyzerTask.hasMirrorUsage(library)
+              ? MessageKind.MIRROR_IMPORT
+              : MessageKind.MIRROR_IMPORT_NO_USAGE;
           compiler.withCurrentElement(library, () {
-            compiler.reportInfo(importTag, MessageKind.MIRROR_IMPORT);
+            compiler.reportInfo(importTag, kind);
           });
         }
       }
     }
   }
 
   Element getDartClass(Element element) {
     for (ClassElement dartClass in implementationClasses.keys) {
       if (element == implementationClasses[dartClass]) {
         return dartClass;
@@ skipping 655 matching lines @@
     copy(constant.values);
     copy(constant.protoValue);
     copy(constant);
   }
 
   void visitConstructed(ConstructedConstant constant) {
     copy(constant.fields);
     copy(constant);
   }
 }