Create class AST nodes.

dart/frog/leg/tree/nodes.dart

Index: dart/frog/leg/tree/nodes.dart
diff --git a/dart/frog/leg/tree/nodes.dart b/dart/frog/leg/tree/nodes.dart
index 569f9c59d60a09f20a9c58d33937eb29d1327be6..41d47b4f1cfb717d1af624fce7b393cee164a7a6 100644
--- a/dart/frog/leg/tree/nodes.dart
+++ b/dart/frog/leg/tree/nodes.dart
@@ -69,14 +69,22 @@ class Node implements Hashable {
 
 class ClassNode extends Node {
   final Identifier name;
+  final TypeAnnotation superclass;
+  final NodeList interfaces;
 
   final Token beginToken;
+  final Token extendsKeyword;
   final Token endToken;
 
-  const ClassNode(this.name, this.beginToken, this.endToken);
+  const ClassNode(this.name, this.superclass, this.interfaces,
+                  this.beginToken, this.extendsKeyword, this.endToken);
 
   accept(Visitor visitor) => visitor.visitClassNode(this);
 
+  bool get isInterface() => beginToken.stringValue === 'interface';
+
+  bool get isClass() => !isInterface;
+
   Token getBeginToken() => beginToken;
 
   Token getEndToken() => endToken;