Handle modifiers (abstract) on class nodes.

lib/compiler/implementation/dart_backend/utils.dart

Index: lib/compiler/implementation/dart_backend/utils.dart
diff --git a/lib/compiler/implementation/dart_backend/utils.dart b/lib/compiler/implementation/dart_backend/utils.dart
index 2086a544de31578a86b3249e62f6fa02fc7d001c..0563979d5dc64d256b3be14b2cb8c106c4732f8a 100644
--- a/lib/compiler/implementation/dart_backend/utils.dart
+++ b/lib/compiler/implementation/dart_backend/utils.dart
@@ -44,7 +44,7 @@ class CloningVisitor implements Visitor<Node> {
       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);