Reapply "Add "load from .dill" file capability and run a white-box test.""

tests/compiler/dart2js/analyze_unused_dart2js_test.dart

 // Copyright (c) 2013, 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 analyze_unused_dart2js;
 
 import 'package:async_helper/async_helper.dart';
 
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/diagnostics/messages.dart';
+import 'package:compiler/src/elements/elements.dart' show LibraryElement;
 import 'package:compiler/src/filenames.dart';
 
 import 'analyze_helper.dart';
 
 // Do not remove WHITE_LIST even if it's empty.  The error message for
 // unused members refers to WHITE_LIST by name.
 const Map<String, List<String>> WHITE_LIST = const {
   // Helper methods for debugging should never be called from production code:
   "lib/src/helpers/": const [" is never "],
 
@@ skipping 52 matching lines @@
         member.forEachLocalMember(checkLive);
       }
     } else if (member.isTypedef) {
       if (member.isResolved) {
         compiler.reporter.reportHintMessage(member, MessageKind.GENERIC,
             {'text': "Helper typedef in production code '$member'."});
       }
     }
   }
 
-  compiler.libraryLoader.lookupLibrary(helperUri).forEachLocalMember(checkLive);
+  (compiler.libraryLoader.lookupLibrary(helperUri) as LibraryElement)
+      .forEachLocalMember(checkLive);
   return handler.checkResults();
 }