Add support for async validations about modddels HOT 1 OPEN

Right now I really don't know which way we should go.

• Option 1 : For the builder, we can either generate it, or make it the annotated class itself. If we generate it, the factory constructors should be made private so that they're never called directly (it's kinda weird). If we make it the annotated class itself, then devs won't be able to add members (methods, getters...) to the modddel (they can only add instance members using extensions).
• Option 2: Init methods are unsafe, unless a lint is added to meta (See dart-lang/linter#4514)
• Option 3: Static methods are very verbose when it comes to generics (which I plan to add support for : #18). Also, I'm not a fan of having "create" in the name.

There will be other cleaner solutions once augmentation libraries are released, but that will take a while.

from modddels.