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: }
  
  /// The runes (integer Unicode code points) of a [String].
  class Runes extends Iterable<int> {
    /// The string that this is the runes of.
    final String string;
  
negative.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:
  