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 fff93e450b05f3ac76f218898bbfbd2c97a66121..d8c9e6d97fef7ba42a3982de8fc5df5199094d17 100644
--- a/lib/compiler/implementation/tree/nodes.dart
+++ b/lib/compiler/implementation/tree/nodes.dart
@@ -172,6 +172,7 @@ class Node implements Spannable {
 }
 
 class ClassNode extends Node {
+  final Modifiers modifiers;
   final Identifier name;
   final TypeAnnotation superclass;
   final NodeList interfaces;
@@ -185,9 +186,9 @@ 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;