assertThrows(NullPointerException.class, () -> builder.put(null, 1));
    builder.put("foo", 2);
    assertMapEquals(builder.buildOrThrow(), "foo", 2);
  }

  public void testBuilderPutImmutableEntryWithNullKeyFailsAtomically() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(NullPointerException.class, () -> builder.put(immutableEntry((String) null, 1)));
    builder.put("foo", 2);