code: public void assertIsDirectory(Description description, File actual) {
      assertNotNull(description, actual);
      if (!actual.isDirectory()) {
        String format = "expecting path:<%s> to represent an existing directory";
        throw failures.failure(description, new BasicErrorMessageFactory(format, actual));
      }
    }
docs: Asserts that the <em>actual</em> {@code File} represents an existing directory.
  
  @param description the description of the <em>actual</em> {@code File}.
  @param actual the <em>actual</em> {@code File}.
  @throws AssertionError if the <em>actual</em> {@code File} is {@code null}.
  @throws AssertionError if the <em>actual</em> {@code File} does not represent an existing directory.