anchor.diff:    ///
     /// Calls the `setter` callback with the new value when the
     /// service extension method is called with a new value.
  +  @protected
     void registerBoolServiceExtension({
       String name,
       @required ValueGetter<bool> getter,
  
positive.code:   /// no value.
    ///
    /// Calls the `callback` callback when the service extension is called.
    void registerSignalServiceExtension({
      @required String name,
      @required VoidCallback callback
  
negative.code:   ///
    /// Calls the `setter` callback with the new value when the
    /// service extension method is called with a new value.
    @protected
    void registerNumericServiceExtension({
      @required String name,
      @required ValueGetter<double> getter,
  