Issue 3931. Visit children nodes safely.

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

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 package com.google.dart.compiler.ast;
 
 /**
  * Represents an entry in a Dart map literal value.
  */
 public class DartMapLiteralEntry extends DartNode {
 
   private DartExpression key;
   private DartExpression value;
 
   public DartMapLiteralEntry(DartExpression key, DartExpression value) {
     this.key = becomeParentOf(key);
     this.value = becomeParentOf(value);
   }
 
   public DartExpression getKey() {
     return key;
   }
 
   public DartExpression getValue() {
     return value;
   }
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {
-    key.accept(visitor);
-    value.accept(visitor);
+    safelyVisitChild(key, visitor);
+    safelyVisitChild(value, visitor);
   }
 
   @Override
   public <R> R accept(ASTVisitor<R> visitor) {
     return visitor.visitMapLiteralEntry(this);
   }
 }