dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/library_loader.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 dart2js.library_loader;
 
 import 'dart:async';
 
-import 'common.dart';
 import 'common/names.dart' show Uris;
 import 'common/tasks.dart' show CompilerTask;
+import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'elements/elements.dart'
     show
         CompilationUnitElement,
         Element,
         ImportElement,
         ExportElement,
-        LibraryElement,
-        PrefixElement;
+        LibraryElement;
 import 'elements/modelx.dart'
     show
         CompilationUnitElementX,
         DeferredLoaderGetterElementX,
         ErroneousElementX,
         ExportElementX,
         ImportElementX,
         LibraryElementX,
         LibraryDependencyElementX,
         PrefixElementX,
@@ skipping 1444 matching lines @@
   /// Called after a request to load a library. The [results] will include all
   /// transitive libraries loaded as a result of the initial request.
   Future onLibrariesLoaded(LoadedLibraries results);
 
   /// Called whenever a library element is created.
   void onLibraryCreated(LibraryElement library);
 
   /// Called whenever a library is scanned from a script file.
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader);
 }