Issue 3931. Visit children nodes safely.

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

Index: compiler/java/com/google/dart/compiler/ast/DartCatchBlock.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartCatchBlock.java b/compiler/java/com/google/dart/compiler/ast/DartCatchBlock.java
index f1833e4d89b022f695e9123f4d295d6e0ac4122b..fb14eef80cdac046de927936afe9948341a2986d 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartCatchBlock.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartCatchBlock.java
@@ -34,11 +34,9 @@ public class DartCatchBlock extends DartStatement {
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {
-    exception.accept(visitor);
-    if (stackTrace != null) {
-      stackTrace.accept(visitor);
-    }
-    block.accept(visitor);
+    safelyVisitChild(exception, visitor);
+    safelyVisitChild(stackTrace, visitor);
+    safelyVisitChild(block, visitor);
   }
 
   @Override