Use ASTVisitor as in new AST.

compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java

Index: compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java b/compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java
index bc6873f2d38399dc0f373101ccd0fd5fed57e9c0..0a6486b0637f20393b9a603db1898a302af8b0bb 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartNullLiteral.java
@@ -17,17 +17,11 @@ public class DartNullLiteral extends DartLiteral {
   }
 
   @Override
-  public void traverse(DartVisitor v, DartContext ctx) {
-    v.visit(this, ctx);
-    v.endVisit(this, ctx);
+  public void visitChildren(ASTVisitor<?> visitor) {
   }
 
   @Override
-  public void visitChildren(DartPlainVisitor<?> visitor) {
-  }
-
-  @Override
-  public <R> R accept(DartPlainVisitor<R> visitor) {
+  public <R> R accept(ASTVisitor<R> visitor) {
     return visitor.visitNullLiteral(this);
   }
 }