dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/serialization/resolved_ast_serialization.dart

 // Copyright (c) 2016, 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.serialization.resolved_ast;
 
 import '../common.dart';
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../diagnostics/diagnostic_listener.dart';
 import '../elements/elements.dart';
-import '../parser/parser.dart' show Parser;
 import '../parser/listener.dart' show ParserError;
 import '../parser/node_listener.dart' show NodeListener;
+import '../parser/parser.dart' show Parser;
 import '../resolution/enum_creator.dart';
 import '../resolution/send_structure.dart';
 import '../resolution/tree_elements.dart';
+import '../tokens/token.dart';
 import '../tree/tree.dart';
-import '../tokens/token.dart';
 import '../universe/selector.dart';
 import 'keys.dart';
 import 'serialization.dart';
 import 'serialization_util.dart';
 
 /// Visitor that computes a node-index mapping.
 class AstIndexComputer extends Visitor {
   final Map<Node, int> nodeIndices = <Node, int>{};
   final List<Node> nodeList = <Node>[];
 
@@ skipping 416 matching lines @@
             objectDecoder.getObject(Key.NEW_STRUCTURE, isOptional: true);
         if (newStructureDecoder != null) {
           elements.setNewStructure(
               node, deserializeNewStructure(newStructureDecoder));
         }
       }
     }
     return new ParsedResolvedAst(element, root, elements);
   }
 }