Remove $Subtype and $Supertype when converting

[Li0liQ]

$Subtype and $Shape generic flow types don't make sense in typescript.
The tool should remove them, i.e. $Subtype<Type> -> Type.

See microsoft/TypeScript#2710 (comment) .

[khell]

$Shape is an alias of Partial: https://github.com/piotrwitek/utility-types/blob/master/src/utility-types.ts#L134 In Flow it marks all the properties of a type optional, likewise with its TS equivalent. Can you please elaborate on why it doesn't make sense? Maybe we should change it directly to Partial and skip the utility-types alias.

Agreed with $Subtype. I don't think that's something we currently handle at all so it is output as-is. I'll add a case for it.

[khell]

Completed here, thanks!
#8