vim-patch:8.1.2177: Dart files are not recognized

Problem:    Dart files are not recognized.
Solution:   Add a filetype rule. (Eugene Ciurana, closes vim/vim#5087)
afbdb905c3

src/nvim/testdir/test_filetype.vim

@@ -122,6 +122,7 @@ let s:filename_checks = {
     \ 'cvs': ['cvs123'],
     \ 'cvsrc': ['.cvsrc'],
     \ 'cynpp': ['file.cyn'],
+    \ 'dart': ['file.dart', 'file.drt'],
     \ 'datascript': ['file.ds'],
     \ 'dcd': ['file.dcd'],
     \ 'debcontrol': ['/debian/control'],