Add ImportElement and ExportElement

pkg/compiler/lib/src/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 1502 matching lines @@
     if (totalMethodCount != preMirrorsMethodCount) {
       int mirrorCount = totalMethodCount - preMirrorsMethodCount;
       double percentage = (mirrorCount / totalMethodCount) * 100;
       compiler.reportHint(
           compiler.mainApp, MessageKind.MIRROR_BLOAT,
           {'count': mirrorCount,
            'total': totalMethodCount,
            'percentage': percentage.round()});
       for (LibraryElement library in compiler.libraryLoader.libraries) {
         if (library.isInternalLibrary) continue;
-        for (LibraryTag tag in library.tags) {
-          Import importTag = tag.asImport();
-          if (importTag == null) continue;
-          LibraryElement importedLibrary = library.getLibraryFromTag(tag);
+        for (ImportElement import in library.imports) {
+          LibraryElement importedLibrary = import.importedLibrary;
           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, kind);
+            compiler.reportInfo(import, kind);
           });
         }
       }
     }
     return programSize;
   }
 
   Element getDartClass(Element element) {
     for (ClassElement dartClass in implementationClasses.keys) {
       if (element == implementationClasses[dartClass]) {
@@ skipping 1613 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }