assertEquals("b", consumingIterator.next());
    assertThat(list).isEmpty();

    assertFalse(consumingIterator.hasNext());
  }

  @GwtIncompatible // ?
  // TODO: Figure out why this is failing in GWT.
  public void testConsumingIterator_duelingIterators() {
    // Test data
    List<String> list = Lists.newArrayList("a", "b");

    // Test & Verify

    return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
  }

  /**
   * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
   * referenced by emptyModifiableIterator().
   */
  private enum EmptyModifiableIterator implements Iterator<Object> {
    INSTANCE;

    @Override
    public boolean hasNext() {

  }

  void assertThatFutureFailsWithException(Future<?> future) {
    try {
      getUninterruptibly(future);
      fail("Expected future to fail: " + future);
    } catch (ExecutionException e) {
      assertThat(e).hasCauseThat().isSameInstanceAs(exception);
    }
  }

  static void assertThatFutureBecomesCancelled(Future<?> future) throws ExecutionException {
    try {

    return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
  }

  /**
   * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
   * referenced by emptyModifiableIterator().
   */
  private enum EmptyModifiableIterator implements Iterator<Object> {
    INSTANCE;

    @Override
    public boolean hasNext() {

   * asList}{@code (...))}, or for creating an empty set then calling {@link Collections#addAll}.
   * This method is not actually very useful and will likely be deprecated in the future.
   */
  @SuppressWarnings("nullness") // TODO: b/316358623 - Remove after checker fix.
  public static <E extends @Nullable Object> HashSet<E> newHashSet(E... elements) {

    assertEquals("b", consumingIterator.next());
    assertThat(list).isEmpty();

    assertFalse(consumingIterator.hasNext());
  }

  @GwtIncompatible // ?
  // TODO: Figure out why this is failing in GWT.
  public void testConsumingIterator_duelingIterators() {
    // Test data
    List<String> list = Lists.newArrayList("a", "b");

    // Test & Verify

      Type subtypeParam = getCovariantTypeResolver().resolveType(typeVars[i]);
      // If 'supertype' is "List<? extends CharSequence>"
      // and 'this' is StringArrayList,
      // First step is to figure out StringArrayList "is-a" List<E> where <E> = String.
      // String is then matched against <? extends CharSequence>, the supertypeArgs[0].
      if (!of(subtypeParam).is(supertypeArgs[i], typeVars[i])) {
        return false;

          // misconfigured mock. Try to switch to Failure.
          Failure failure;
          try {
            failure = new Failure(t);
          } catch (Exception | Error oomMostLikely) { // sneaky checked exception
            failure = Failure.FALLBACK_INSTANCE;
          }

        setSystemProperty(Constants.IGNORE_FAILURE_TYPE_PROPERTY, value);
    }

    default String getIgnoreFailureType() {
        return getSystemProperty(Constants.IGNORE_FAILURE_TYPE_PROPERTY, Constants.DEFAULT_IGNORE_FAILURE_TYPE);
    }

    default void setFailureCountThreshold(final int value) {
        setSystemPropertyAsInt(Constants.FAILURE_COUNT_THRESHOLD_PROPERTY, value);
    }

 * environment if possible) before spending a lot of effort on tweaking a particular element.
 *
 * <h3>Other types of preconditions</h3>
 *
 * <p>Not every type of precondition failure is supported by these methods. Continue to throw
 * standard JDK exceptions such as {@link java.util.NoSuchElementException} or {@link
 * UnsupportedOperationException} in the situations they are intended for.
 *