Fix missing import (caused by failed merge.)

core/src/test/java/hudson/UtilTest.java

@@ -30,6 +30,7 @@ import java.util.HashMap;
 import java.util.Locale;
 import java.util.Properties;
 import java.util.concurrent.Callable;
+import java.util.concurrent.atomic.AtomicReference;
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.FileInputStream;