anchor.diff:  
     Object _optionalPositional;
     static JSArray _getOptionalPositional(_FunctionParameters parameters) =>
  -      JS('JSArray', '#', parameters._optionalPositional);
  +      JS('JSUnmodifiableArray', '#', parameters._optionalPositional);
     static void _setOptionalPositional(
         _FunctionParameters parameters, Object optionalPositional) {
       parameters._optionalPositional = optionalPositional;
  
positive.code:   /// Invariant: These pairs are sorted by name in lexicographically ascending order.
    Object _optionalNamed;
    static JSArray _getOptionalNamed(_FunctionParameters parameters) =>
        JS('JSArray', '#', parameters._optionalNamed);
    static void _setOptionalNamed(
        _FunctionParameters parameters, Object optionalNamed) {
      parameters._optionalNamed = optionalNamed;
  
negative.code:   /// - Class part of a type environment inside a generic class, or `null` for
    ///   type tuple.
    /// - Return type of a function type.
    /// - Underlying function type for a generic function.
    /// - de Bruijn index for a generic function parameter.
    dynamic _primary;
  
    static Object _getPrimary(Rti rti) => rti._primary;
  