anchor.diff:  /// A constant, absent Optional.
   const Optional<dynamic> empty = _Absent<dynamic>();
   
  -/**
  - * A container object which may contain a non-null value.
  - *
  - * Offers several methods which depend on the presence or absence of a contained value.
  - */
  +/// A container object which may contain a non-null value.
  +///
  +/// Offers several methods which depend on the presence or absence of a contained value.
   abstract class Optional<T> {
  -  /**
  -   * Creates a new Optional with the given non-null value.
  -   *
  -   * Throws [ArgumentError] if value is null.
  -   */
  +  /// Creates a new Optional with the given non-null value.
  +  ///
  +  /// Throws [ArgumentError] if value is null.
     factory Optional.of(T value) {
       if (value == null) {
         throw ArgumentError('value must be non-null');
  
positive.code:   /// 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.
     */
    Optional<R> map<R>(R mapper(T val));
  
    /// Returns this Optional's value, if present.  Otherwise, returns other.
  
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();
  