dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/tree/unparser.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 '../tokens/token.dart' show BeginGroupToken, Token;
+import '../tokens/token.dart' show Token;
 import '../tokens/token_constants.dart' as Tokens
     show IDENTIFIER_TOKEN, KEYWORD_TOKEN, PLUS_TOKEN;
 import '../util/util.dart';
 import 'nodes.dart';
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
 }
@@ skipping 880 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }