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

  /**
   * Returns the result of calling {@link Future#get()} uninterruptibly on a task known not to throw
   * a checked exception. This makes {@code Future} more suitable for lightweight, fast-running
   * tasks that, barring bugs in the code, will not fail. This gives it exception-handling behavior
   * similar to that of {@code ForkJoinTask.join}.
   *
   * <p>Exceptions from {@code Future.get} are treated as follows:
   *
   * <ul>

    } catch (NullPointerException expected) {
    }
    ImmutableList<String> result = builder.build();

    /*
     * Maybe it rejects all elements, or maybe it adds "a" before failing.
     * Either way is fine with us.
     */
    if (result.isEmpty()) {
      return;
    }
    assertTrue(ImmutableList.of("a").equals(result));
    assertEquals(1, result.size());
  }