Move Compiler.libraries to LibraryLoader.

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/implementation/dart2jslib.dart';
 import 'package:compiler/implementation/filenames.dart';
@@ skipping 38 matching lines @@
       } else {
         member.forEachLocalMember(checkLive);
       }
     } else if (member.isTypedef) {
       if (member.isResolved) {
         compiler.reportHint(member, MessageKind.GENERIC,
             {'text': "Helper typedef in production code '$member'."});
       }
     }
   }
-  compiler.libraries['$helperUri'].forEachLocalMember(checkLive);
+  compiler.libraryLoader.lookupLibrary(helperUri).forEachLocalMember(checkLive);
   return handler.checkResults();
 }