patch 8.2.4167: Vim9: error message for old style import

Problem:    Vim9: error message for old style import.
Solution:   Use another error message.  Add a test.
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index b15d411..bacba2e 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -429,6 +429,19 @@
       var that = foo
   END
   CheckScriptFailure(lines, 'E1060: Expected dot after name: foo')
+  lines =<< trim END
+      vim9script
+      import './Xfoo.vim' as foo
+      var that: any
+      that += foo
+  END
+  CheckScriptFailure(lines, 'E1060: Expected dot after name: foo')
+  lines =<< trim END
+      vim9script
+      import './Xfoo.vim' as foo
+      foo += 9
+  END
+  CheckScriptFailure(lines, 'E1060: Expected dot after name: foo')
 
   lines =<< trim END
       vim9script