make frontend strong clean

[sigmundch]

This is a meta-bug to track what needs to be done to make package:front_end strong clean.

General errors pending to be fixed in package:front_end:

• Incorrect type annotations
• Covariant overrides
• Covariant closures to forEach
• Downcast composite warnings

Issues that should be fixed in the language:

• bad interaction: implicit downcast + inferred instantiated guarded type args #28646
• Infer cases where @virtual is implicit #28601

This CL will fix the first 3 and temporarily ignore all the downcast composite warnings. The downward composite warnings are clearly cases that will fail at runtime in strong mode, so we can't ignore them indefinitely. There are a few changes that will help fix them:

• add generics to stack_listener push pop and popList
• avoid union types:
  • split optional arguments as a separate stack entry (see outline_builder)
  • see if we can flatten nested statement blocks differently (see body_builder).

We are delaying those changes until we set up some infrastructure that lets us track if those changes have any visible runtime performance effect.

[kasperl]

Fields are now virtual by defaults and the composite downcast warnings are opt-in.

[mraleph]

I am pretty sure this is done by now.