Issue 3931. Visit children nodes safely.

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

Index: compiler/java/com/google/dart/compiler/ast/DartImportDirective.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartImportDirective.java b/compiler/java/com/google/dart/compiler/ast/DartImportDirective.java
index 9a849f91e1528065991dae47562ba09f7f37cd4f..d4cc9d9f2be855f3a5b8e529816d56fa69d98157 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartImportDirective.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartImportDirective.java
@@ -1,4 +1,4 @@
-// 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.
 
@@ -43,14 +43,10 @@ public class DartImportDirective extends DartDirective {
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {
-    libraryUri.accept(visitor);
-    if (exported != null) {
-      exported.accept(visitor);
-    }
+    safelyVisitChild(libraryUri, visitor);
+    safelyVisitChild(exported, visitor);
     combinators.accept(visitor);
-    if (prefix != null) {
-      prefix.accept(visitor);
-    }
+    safelyVisitChild(prefix, visitor);
   }
 
   @Override