dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/tree/nodes.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.
 
 import 'dart:collection' show IterableMixin;
 
 import '../common.dart';
+import '../elements/elements.dart' show MetadataAnnotation;
+import '../resolution/secret_tree_element.dart'
+    show NullTreeElementMixin, StoredTreeElementMixin;
 import '../tokens/precedence_constants.dart' as Precedence show FUNCTION_INFO;
 import '../tokens/token.dart' show BeginGroupToken, Token;
-import '../tokens/token_constants.dart' as Tokens
-    show IDENTIFIER_TOKEN, KEYWORD_TOKEN, PLUS_TOKEN;
+import '../tokens/token_constants.dart' as Tokens show PLUS_TOKEN;
+import '../util/characters.dart';
 import '../util/util.dart';
-import '../util/characters.dart';
-import '../resolution/secret_tree_element.dart'
-    show NullTreeElementMixin, StoredTreeElementMixin;
-import '../elements/elements.dart' show MetadataAnnotation;
 import 'dartstring.dart';
 import 'prettyprint.dart';
 import 'unparser.dart';
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
 
   R visitAssert(Assert node) => visitStatement(node);
@@ skipping 3087 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }