anchor.diff:  
     int get last {
       if (string.length == 0) {
  -      throw new StateError("No elements.");
  +      throw new StateError('No elements.');
       }
       int length = string.length;
       int code = string.codeUnitAt(length - 1);
  
positive.code:     if (index > 0 && index < string.length &&
          _isLeadSurrogate(string.codeUnitAt(index - 1)) &&
          _isTrailSurrogate(string.codeUnitAt(index))) {
        throw new ArgumentError("Index inside surrogate pair: $index");
      }
    }
  
  
negative.code:   /**
     * Gets the character (as a single-code-unit [String]) at the given [index].
     *
     * The returned string represents exactly one UTF-16 code unit, which may be
     * half of a surrogate pair. A single member of a surrogate pair is an
     * invalid UTF-16 string:
     *
     *     var clef = '\u{1D11E}';
     *     // These represent invalid UTF-16 strings.
     *     clef[0].codeUnits;      // [0xD834]
     *     clef[1].codeUnits;      // [0xDD1E]
     *
     * This method is equivalent to
     * `new String.fromCharCode(this.codeUnitAt(index))`.
  