Currently, on some platforms providing XTypeGroup extensions with the . will work, and on others it won't; see flutter/devtools#2506 (comment) for an example of confusion caused by that.

We should strongly consider normalizing the extensions in the Dart side of the plugin (or maybe in XTypeGroup itself) to the non-. form (since that's what an extension list should have; the extension of foo.txt is txt, not .txt), so that we don't end up with different behavior on different platforms when people pass the wrong version.

/cc @tugorez, @ditman