anchor.diff:     * [start] and increasing [end], up to at most length characters.
      * If the start or end of the slice are not matching the start or end of
      * the string, ellipses are added before or after the slice.
  -   * Control characters may be encoded as "\xhh" codes.
  +   * Characters other than printable ASCII are escaped.
      */
     static String _truncateString(String string, int start, int end, int length) {
       if (end - start > length) {
  
positive.code:   static void identical(var expected, var actual, [String reason = null]) {
      if (_identical(expected, actual)) return;
      String msg = _getMessage(reason);
      _fail("Expect.identical(expected: <$expected>, actual: <$actual>$msg) "
          "fails.");
    }
  
negative.code:       if (stringDifference != null) {
          _fail("Expect.equals($stringDifference$msg) fails.");
        }
        _fail("Expect.equals(expected: <${_escapeString(expected)}>"
            ", actual: <${_escapeString(actual)}>$msg) fails.");
      }
      _fail("Expect.equals(expected: <$expected>, actual: <$actual>$msg) fails.");
    }
  