Allow generic function types as type arguments

[jcollins-g]

Blocks dart-lang/sdk#44922.

Implement dart-lang/language#496 for dartdoc once analyzer support is available.

[jcollins-g]

Blocked by dart-lang/sdk#44929.

[leafpetersen]

How's this looking?

[jcollins-g]

This is crashing with current published analyzer. It does not crash, but has some obvious errors/omissions particularly in type bounds for generated documentation at head.

[franklinyow]

Any update?

[devoncarew]

@jcollins-g - FYI, the likely branch point for the next beta release (the first with this language feature enabled) is on or around May 18th; this is the last bit of work left for the small language features release.

[jcollins-g]

@devoncarew is that May 18th date still accurate?