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 85b6cb1ff34fbca0eaf2a4328d1b1e298cd8ef18..5edef55bdfd8b0bb545ebcd2b17c6638c3bf29ef 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;