dillon1000/react

[be] Clean up null -> PolyType in ObjectShape

--- 

Question - should the unifier ignore `PolyType` for now? 

```js 

class Unifier { 

// ... 

bindVariableTo(v: TypeVar, type: Type) { 

if (type.kind === "Poly") { 

return; 

} 

// ... 

```