the modules that the test module classes use, so we get errors.

            The good news is that, despite ignoring errors here, GWT does fail if any errors affect
            classes that are actually used in the module under test.

            One way to eliminate the warnings is to make base.testModule include the not really

   * fewer than {@code k} have been offered, in the order specified by the factory used to create
   * this {@code TopKSelector}.
   *
   * <p>The returned list is an unmodifiable copy and will not be affected by further changes to
   * this {@code TopKSelector}. This method returns in O(k log k) time.
   */
  public List<T> topK() {
    @SuppressWarnings("nullness") // safe because we pass sort() a range that contains real Ts