Upgrade Flutter and add null safety support

Hello @avbk

I was also including "create issue" support when I encountered your MR. I noticed that your merge request (exitlive/gitlab-dart!11) was never merged, so I decided to open a merge request over your develop to avoid duplicated work.

Basically this merge request update the code to run with "null safety". It runs the automatic conversion with some fix on the top of it. I have also updates mockito to generate the test class according to its new documentation.

Would you like to review/merge this?

Ian