Use Flutter stable for testing, format, and analysis

[creativecreatorormaybenot]

The analyisis (and other) runs on flutter channel master which causes problems:

• [animations] fix invalid text style names #103
• [animations] Remove unnecessary override #106

Both of these are failing in CI checks, even though they would not fail on stable. This also means that something like the animations example does not even work on stable because it uses members from master. Still, the checks did not complain.

[goderbauer]

I don't think we should only test on stable. It will be important for us to also know that these work with master...

[creativecreatorormaybenot]

@goderbauer I agree. Still, is it not more important that they run on stable primarily? The example (#103) does not run on stable at all.

[goderbauer]

I don't think any of these channels is more important than any of the others. Many people ship their app from the dev or master channel...

[gspencergoog]

@creativecreatorormaybenot, I'm going to close this PR not because your point isn't valid (it is!), but because we think this particular approach would cause more problems than it would solve right now. Instead, if you could, please file an issue that describes the problem and we'll try and find a more universal solution.

[creativecreatorormaybenot]

@gspencergoog flutter/flutter#51540