To better support analyzer plugins on custom file endings, I've made two changes:

1. Register providers that aren't specific to Dart (all those we register on MODE_HTML) for extensions declared in additionalAnalyzerFileExtensions if the option to use the Angular plugin is turned on.
2. Use the analysis server folding for both html and custom files if enabled.

With a plugin I'm writing, I could verify that auto-complete, folding, outline, problems, fixes and navigation are all working with this setup.

Fixes #1981

Potential problems

In the linked issue, you asked whether the analysis server would send problematic responses when used with unsupported files. I played around with this as the plugin was turned off, and I didn't run into any problems.
Further, that shouldn't have any impact on the first change because it only effects users who explicitly added a file to additionalAnalyzerFileExtensions, which only makes sense when an appropriate plugin is available.