Fix dart2js warnings and add test to ensure it stays clean.

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/names.dart' show Uris;
 import 'common/tasks.dart' show CompilerTask, Measurer;
@@ skipping 18 matching lines @@
         SyntheticImportElement;
 import 'enqueue.dart' show DeferredAction;
 import 'environment.dart';
 import 'patch_parser.dart' show PatchParserTask;
 import 'resolved_uri_translator.dart';
 import 'script.dart';
 import 'serialization/serialization.dart' show LibraryDeserializer;
 import 'tree/tree.dart';
 import 'util/util.dart' show Link, LinkBuilder;
 
-import 'package:front_end/src/fasta/scanner.dart' show Token;
-
 typedef Future<Iterable<LibraryElement>> ReuseLibrariesFunction(
     Iterable<LibraryElement> libraries);
 
 typedef Uri PatchResolverFunction(String dartLibraryPath);
 
 /**
  * [CompilerTask] for loading libraries and setting up the import/export scopes.
  *
  * The library loader uses four different kinds of URIs in different parts of
  * the loading process.
@@ skipping 1501 matching lines @@
 }
 
 /// API used by the library loader to synchronously scan a library or
 /// compilation unit and ensure that their library tags are computed.
 abstract class ElementScanner {
   void scanLibrary(LibraryElement library);
   void scanUnit(CompilationUnitElement unit);
 }
 
 const _reuseLibrarySubtaskName = "Reuse library";