compiler.cljc : `is-assignable` returns true for Function when it should not

[dupuchba]

to reproduce:

(binding [analyzer-info li
            *dart-out* *out*
            *locals-gen* {}]
    (is-assignable?      
      (emit-type '^{:params-types (String)} dart:core/Function {})
      (emit-type '^{:params-types (int)} dart:core/Function {})))

It has impact in many places of the code but I discovered it when trying a (dart/is? f #/(-> int)) that should return false

[cgrand]

@dupuchba didn't you work on it?

[dupuchba]

I've worked on this but it's not finished