Incremental reflectable builds are very slow

A few weeks ago I noticed, that our reflectable builds were slowing down (incremental as well as non-incremental): https://github.com/dart-lang/reflectable/issues/188#issuecomment-600619300

In the beginning, the incremental builds were just taking a few seconds (5 seconds) longer, but as the project progressed, those incremental builds were getting slower and slower (25s-30s). Now they it becomes quite unmanageable. You can't just hit the run test button, you always need to wait for 30s to start the test again, which makes TDD impossible.

A few things that might help to resolve this:

• The project consists of 4 dart packages, each one has one entry point for tests. And one package is our actual flutter app, which has an additional entry point for the main.dart.
• The project size is moderate, nothing special
• The main.reflectable.dart (fazua_toolbox_app) for example (Flutter entry point) has 19222 lines and 536944 characters
• It is problem is os independent, means it occurs on Linux, macOS and Windows
• It feels like every incremental build throws away most of the cache and starts rebuilding more or less everything

I also opend an issue on the reflectable repo: https://github.com/dart-lang/reflectable/issues/207