anchor.diff:  /// Clients may not extend, implement or mix-in this class.
   abstract class DirectiveUriWithRelativeUri
       extends DirectiveUriWithRelativeUriString {
  +  /// The relative URI, parsed from [relativeUriString].
     Uri get relativeUri;
   }
   
  
positive.code: ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class DirectiveUriWithLibrary extends DirectiveUriWithSource {
    LibraryElement get library;
  }
  
  
negative.code: ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class DirectiveUriWithUnit extends DirectiveUriWithSource {
    /// The unit referenced by the [source].
    CompilationUnitElement get unit;
  }
  
  