Eliminate type checks in field setters

[askeksa-google]

This CL adds missing visitor methods in the TFA transformer. The added handling of implicit setters

        typeChecksNeeded = field.isGenericCovariantImpl;

is more conservative than it needs to be, as it doesn't take information collected by the TFA into account.

Interestingly, the change causes a slight code size regression in Flutter Gallery on ARMv7. Adding more TFA information ought to only improve the compilation, but maybe some inlining instability is at play here.

[alexmarkov]

/cc @sjindel-google

[mkustermann]

Interestingly, the change causes a slight code size regression in Flutter Gallery on ARMv7. Adding
more TFA information ought to only improve the compilation, but maybe some inlining instability is
at play here.

/cc @sstrickl (for inlining investigations)