Fix warnings in dart2js.

dart/lib/compiler/implementation/elements/elements.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('elements');
 
 #import('dart:uri');
 
 #import('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
@@ skipping 502 matching lines @@
     }
   }
 }
 
 class LibraryElement extends ScopeContainerElement {
   final Uri uri;
   CompilationUnitElement entryCompilationUnit;
   Link<CompilationUnitElement> compilationUnits =
       const EmptyLink<CompilationUnitElement>();
   Link<LibraryTag> tags = const EmptyLink<LibraryTag>();
-  LibraryTag libraryTag;
+  LibraryName libraryTag;
   bool canUseNative = false;
 
   /**
    * If this library is patched, [patch] points to the patch library.
    *
    * See [:patch_parser.dart:] for a description of the terminology.
    */
   LibraryElement patch = null;
 
   /**
@@ skipping 1339 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }