anchor.diff:    /// result = string.substring(1, 4); // 'art'
     /// ```
     ///
  -  /// Both [start] and [end] must be non-negative and no greater than [length], and
  +  /// Both [start] and [end] must be non-negative and no greater than [length];
     /// [end], if provided, must be greater than or equal to [start].
     String substring(int start, [int? end]);
   
  
positive.code: /// ```
  /// For a character outside the Basic Multilingual Plane (plane 0) that is
  /// composed of a surrogate pair, [runes] combines the pair and returns a
  /// single integer.  For example, the Unicode character for a
  /// musical G-clef ('𝄞') with rune value 0x1D11E consists of a UTF-16 surrogate
  /// pair: `0xD834` and `0xDD1E`. Using [codeUnits] returns the surrogate pair,
  /// and using `runes` returns their combined value:
  
negative.code:   /// The length of the string.
    ///
    /// Returns the number of UTF-16 code units in this string. The number
    /// of [runes] might be fewer if the string contains characters outside
    /// the Basic Multilingual Plane (plane 0):
    /// ```dart
    /// 'Dart'.length;          // 4
  