This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.
This yak shave went as follows:
Fix https://github.com/flutter/flutter/issues/8795 by adding stocks to
the examples README.
Notice the layers entry in that README isn't quite right either.
Update that.
Check the layers/README file is worth pointing at.
Update the layers/README.
Let's run some of the layer tests to see if they still work.
Oops, need to update them to gradle.
Ok let's try running them again.
Oops, sector is broken.
Add a test for sector.
Fix sector. Find you need to add an assert to a const constructor.
Notice we need to turn const asserts on for the analyzer.
Notice the analysis_options files are out of sync with each other and
with the full list of lints.
Turn on the lints that should be on.
Fix the bugs that finds.
