TestCharSource okSource = new TestCharSource(STRING);
    assertThrows(IOException.class, () -> okSource.copyTo(new TestWriter(WRITE_THROWS)));
    assertTrue(okSource.wasStreamClosed());
  }

  public void testConcat() throws IOException {
    CharSource c1 = CharSource.wrap("abc");
    CharSource c2 = CharSource.wrap("");
    CharSource c3 = CharSource.wrap("de");

    String expected = "abcde";