} else {
        assertThat(charset).isAbsent();
      }
    }
  }

  @J2ktIncompatible
  @GwtIncompatible // reflection
  public void testConstants_areUnique() {
    assertThat(getConstants()).containsNoDuplicates();
  }

  @J2ktIncompatible
  @GwtIncompatible // reflection
  private static FluentIterable<Field> getConstantFields() {
    return FluentIterable.from(asList(MediaType.class.getDeclaredFields()))