patch 8.2.2173: Vim9: get internal error when assigning to undefined variable

Problem:    Vim9: get internal error when assigning to undefined variable.
Solution:   Add error message. (closes #7475)
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index bdc9ad0..d297931 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -710,5 +710,16 @@
   CheckScriptFailure(lines, 'E464:')
 enddef
 
+def Test_command_not_recognized()
+  var lines =<< trim END
+    d.key = 'asdf'
+  END
+  CheckDefFailure(lines, 'E1146:', 1)
+
+  lines =<< trim END
+    d['key'] = 'asdf'
+  END
+  CheckDefFailure(lines, 'E1146:', 1)
+enddef
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker