Implement generic function type syntax in dart2js

[floitschG]

This is the dart2js-implementation issue for #27527.

At the moment we only really need the new syntax in typedefs. Eventually, we want to make this syntax usable for fields and locals, but it's not yet as high a priority. If you find the time to make it work there too, please guard it behind a (different) flag.

[mit-mit]

Moving to 1.22 for implementation, informal spec i linked from the meta-issue #27527

[floitschG]

Was reverted. Targetting 1.23 now.