code: private void fixFunctionType(Node functionNode) {
      JSType t = functionNode.getJSType();
      if (t == null) {
        return;
      }
      FunctionType ft = t.toMaybeFunctionType();
      if (ft != null) {
        functionNode.setJSType(convertMethodToFunction(ft));
      }
    }
docs: Creates a new type based on the original function type by
  adding the original this pointer type to the beginning of the
  argument type list and replacing the this pointer type with bottom.