Dual compile reland

[eyebrowsoffire]

This is an attempt at a reland of #141396

The main changes here that are different than the original PR is fixes to wire up the flutter test command properly with the web renderer.

[mdebbar]

I only looked at the 2 commits on top of the previously reverted PR. They look good to me!

[camsim99]

@eyebrowsoffire I suspect that this PR may be blocking the Flutter --> packages roller based on @gspencergoog's findings (see discussion on Discord) on the web test failures in palette_generator (here's an example of a bad build: https://ci.chromium.org/ui/p/flutter/builders/try/Linux_web%20web_dart_unit_test_shard_2%20master/5866/overview).

I think we should revert this PR, but let me know if you think this is a misfire.

[eyebrowsoffire]

@camsim99 Thanks for flagging this. I took a look at the issue, and it appears that those failing tests in the flutter/packages seem to be depending on an actual bug in our code. Not specifying a web renderer in flutter test --platform=chrome should default to the auto selector, but in reality the flutter web platform was previously erroneously serving the files for the html renderer if nothing was specified. I think maybe the best thing here would be to forward fix the packages repo to explicitly specify the web renderer when running flutter test. I will put together a forward fix for that.