Remove generated files and update .gitignore

[madmini]

added some relevant .gitignore entries from https://github.com/flutter/flutter/blob/master/.gitignore

[fzyzcjy]

Good job! However, I am not sure whether that is the best way. For example, in flutter/flutter#13892 there seems to be many possible gitignores. Could you please check whether those entries are all suggested to be there or not?

[madmini]

As the discussion in the linked issue seems to indicate, there is no canonical .gitignore. I believe the one from the main flutter repo may be the closest thing to a complete .gitignore.

As for the entries I copied from there, flutter/flutter#13554 (and the corresponding merged PR) says that these are not intended to be checked in.
In addition, each of these files has a header that says Generated file, do not edit. and is regenerated when running or building the apps.

[fzyzcjy]

Looks reasonable and thanks for the contribution!

[fzyzcjy]

@all-contributors please add @madmini for code

[allcontributors]

@fzyzcjy

@madmini already contributed before to code