Make FunctionType implement TypeReference

[natebosch]

Fixes #174

• Add a test which fails before this change and passes after
• Add tests for nested function types since that was untested previously
• Make FuctionType implement TypeReference, add missing fields
  returning null and rebuild the built_value file.
• Return this from the type getter.

[natebosch]

Hmm, I don't think we can do this. We can't implement both Built<FunctionType, FunctionTypeBuilder> and some other built_value class.

I'm not sure what the right way forward is.

[matanlurey]

What about just loosening whatever is requesting a TypeReference to accept a Reference?

[natebosch]

Changing type to a Reference seems to work.

I think, though, that it's a breaking change. Implementations of SpecVisitor will be broken by the change in signature of visitTypeParameters, and the change of the type field is also technically breaking.

[matanlurey]

Realistically the only implementation of SpecVisitor is DartEmitter today.

... so I'm OK with saying the original implementation was broken, and custom implementations of the visitor/emitter will need a few lines updated to keep working.

[natebosch]

Sounds good. I added a note to the changelog about the breaking change.

[natebosch]

@matanlurey - if you have a chance to publish soon I'd appreciate it 😄

[matanlurey]

@natebosch Definitely today, I'm back home in about an hour.