Handle modifiers (abstract) on class nodes.

lib/compiler/implementation/dart_backend/utils.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.
 
 class CloningVisitor implements Visitor<Node> {
   final TreeElements originalTreeElements;
   final TreeElementMapping cloneTreeElements;
 
   CloningVisitor(this.originalTreeElements)
       : cloneTreeElements = new TreeElementMapping();
@@ skipping 26 matching lines @@
       visit(node.expression), node.cascadeOperator);
 
   visitCaseMatch(CaseMatch node) => new CaseMatch(
       node.caseKeyword, visit(node.expression), node.colonToken);
 
   visitCatchBlock(CatchBlock node) => new CatchBlock(
       visit(node.type), visit(node.formals), visit(node.block),
       node.onKeyword, node.catchKeyword);
 
   visitClassNode(ClassNode node) => new ClassNode(
-      visit(node.name), visit(node.typeParameters),
+      visit(node.modifiers), visit(node.name), visit(node.typeParameters),
       visit(node.superclass), visit(node.interfaces), visit(node.defaultClause),
       node.beginToken, node.extendsKeyword, visit(node.body), node.endToken);
 
   visitConditional(Conditional node) => new Conditional(
       visit(node.condition), visit(node.thenExpression),
       visit(node.elseExpression), node.questionToken, node.colonToken);
 
   visitContinueStatement(ContinueStatement node) => new ContinueStatement(
       visit(node.target), node.keywordToken, node.semicolonToken);
 
@@ skipping 211 matching lines @@
   }
 
   Node visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }