anchor.diff:  ///
   /// Clients may not extend, implement or mix-in this class.
   abstract class DirectiveUriWithSource extends DirectiveUriWithRelativeUri {
  +  /// The result of resolving [relativeUri] against the enclosing URI.
     Source get source;
   }
   
  
positive.code: /// Clients may not extend, implement or mix-in this class.
  abstract class DirectiveUriWithRelativeUri
      extends DirectiveUriWithRelativeUriString {
    Uri get relativeUri;
  }
  
  
negative.code: ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class DirectiveUriWithRelativeUriString extends DirectiveUri {
    /// The relative URI string specified in code.
    String get relativeUriString;
  }
  
  