Use ASTVisitor as in new AST.

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

Index: compiler/java/com/google/dart/compiler/ast/DartAssertion.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartAssertion.java b/compiler/java/com/google/dart/compiler/ast/DartAssertion.java
index b22cb42154ee803acc50b4cb3fb93700a6d9713d..bd030b92bfad04dffdcade607fd71826935adb72 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartAssertion.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartAssertion.java
@@ -23,20 +23,12 @@ public class DartAssertion extends DartStatement {
   }
 
   @Override
-  public void traverse(DartVisitor v, DartContext ctx) {
-    if (v.visit(this, ctx)) {
-      expression = becomeParentOf(v.accept(expression));
-    }
-    v.endVisit(this, ctx);
-  }
-
-  @Override
-  public void visitChildren(DartPlainVisitor<?> visitor) {
+  public void visitChildren(ASTVisitor<?> visitor) {
     expression.accept(visitor);
   }
 
   @Override
-  public <R> R accept(DartPlainVisitor<R> visitor) {
+  public <R> R accept(ASTVisitor<R> visitor) {
     return visitor.visitAssertion(this);
   }
 }