patch 8.1.1816: cannot use a user defined function as a method

Problem:    Cannot use a user defined function as a method.
Solution:   Pass the base as the first argument to the user defined function
            after "->". (partly by FUJIWARA Takuya)
diff --git a/src/testdir/test_autoload.vim b/src/testdir/test_autoload.vim
index 7396c22..b8c4fa2 100644
--- a/src/testdir/test_autoload.vim
+++ b/src/testdir/test_autoload.vim
@@ -8,6 +8,8 @@
   call g:foo#bar.echo()
   call assert_equal(1, g:loaded_foo_vim)
   call assert_equal(1, g:called_foo_bar_echo)
+
+  eval 'bar'->g:foo#addFoo()->assert_equal('barfoo')
 endfunc
 
 func Test_source_autoload()