* not to change during the copy. It is safe to do so in GWT because javascript is
   * single-threaded.
   */
  @GwtIncompatible // GWT is single threaded
  public void testCopyOf_threadSafe() {
    /*
     * The actual collections that we pass as inputs will be wrappers around these, so
     * ImmutableSet.copyOf won't short-circuit because it won't see an ImmutableSet input.
     */