Analyzer should pass experiment flags through to dart_style

[munificent]

The dart_style library API now accepts experiment flags. I added support for that when I made the formatter not hardcode all known experiments. It passes those flags onto the parser when it parses code to format it.

My understanding is that analyzer doesn't currently pass the enabled experiments set in analysis_options.yaml through to the formatter when it formats. It probably should so that the formatter parses the code using the same experiments that analyzer itself is using.