Introduce a method in the Objects utility class, something like:

/**
 * Construct a new instance of an unspecified class. The object has
 * a unique identity—no other references to it exist. It can be
 * used for synchronization, or where a placeholder Object is
 * needed. Use this method to avoid relying on the Object constructor.
 */
public static Object newIdentity() {
    return new Object() {};
}

(The method appears somewhat unmotivated for now, but with JEP 401, use of the Object constructor will be discouraged (with warnings) in Java source, and this method provides a migration path to something else.)