Handle modifiers (abstract) on class nodes.

lib/compiler/implementation/tree/nodes.dart

Index: lib/compiler/implementation/tree/nodes.dart
diff --git a/lib/compiler/implementation/tree/nodes.dart b/lib/compiler/implementation/tree/nodes.dart
index 72467acf9edb737f639d35ab13a9a95ef5871f0a..fe9beb13edef1f911209dddee9df156db7170c03 100644
--- a/lib/compiler/implementation/tree/nodes.dart
+++ b/lib/compiler/implementation/tree/nodes.dart
@@ -201,6 +201,7 @@ abstract class Node implements Spannable {
 }
 
 class ClassNode extends Node {
+  final Modifiers modifiers;
   final Identifier name;
   final TypeAnnotation superclass;
   final NodeList interfaces;
@@ -214,9 +215,9 @@ class ClassNode extends Node {
   final Token extendsKeyword;
   final Token endToken;
 
-  ClassNode(this.name, this.typeParameters, this.superclass, this.interfaces,
-            this.defaultClause, this.beginToken, this.extendsKeyword,
-            this.body, this.endToken);
+  ClassNode(this.modifiers, this.name, this.typeParameters, this.superclass,
+            this.interfaces, this.defaultClause, this.beginToken,
+            this.extendsKeyword, this.body, this.endToken);
 
   ClassNode asClassNode() => this;