I really don’t know. I don’t even have 1.8 here, and have no real experience of it, so cannot guess what you did before that you need to do.
I had a look in Github and the version of AGALMiniAssembler.as in 1.8 was last updated for the then new "standardConstrained" profile, so you are right on that, and 1.8 has support for it. Clearly you have a version of AGALMiniAssembler.as that's aware of AGAL 2 as it’s able to warn you that it thinks you don’t have it.
So it should work, if you have chosen standardConstrained or greater. I can only suggest stepping through it in a debugger and seeing whether that profile is working and being passed to the AGALMiniAssembler correctly.
And also, In future, you could do what I do: back up everything, daily at least. So if you delete or overwrite something and realise too late it contained something important, you can always get an older version from a backup.