throw new AssertionFailedError("shouldn't be invoked");
    }
  }

  @GwtIncompatible // StringBuilder.append in GWT invokes Object.toString(), unlike the JRE version.
  public void testDontConvertCharSequenceToString() {
    assertEquals("foo,foo", Joiner.on(",").join(new DontStringMeBro(), new DontStringMeBro()));
    assertEquals(
        "foo,bar,foo",