anchor.diff:    /// Returns an Optional provided by applying the mapper to this Optional's value, if present.  Otherwise, returns an empty Optional.
     Optional<R> flatMap<R>(Optional<R> mapper(T val));
   
  -  /**
  -   * Returns an Optional containing the result of applying the mapper to this Optional's value, if present.  Otherwise, returns an empty Optional.
  -   *
  -   * If the mapper returns a null value, returns an empty Optional.
  -   */
  +  /// Returns an Optional containing the result of applying the mapper to this Optional's value, if present.  Otherwise, returns an empty Optional.
  +  ///
  +  /// If the mapper returns a null value, returns an empty Optional.
     Optional<R> map<R>(R mapper(T val));
   
     /// Returns this Optional's value, if present.  Otherwise, returns other.
  
positive.code:   /// Creates an empty Optional.
    const factory Optional.empty() = _Absent<T>._internal;
  
    /**
     * The value associated with this Optional, if any.
     *
     * Throws [NoValuePresentError] if no value is present.
     */
    T get value;
  
    /// Whether the Optional has a value.
  
negative.code:     }
    }
  
    /// Creates a new Optional with the given value, if non-null.  Otherwise, returns an empty Optional.
    factory Optional.ofNullable(T value) {
      if (value == null) {
        return empty.cast();
  