[TS] In version 2, why do union helpers use string comparison?

In version 2, union helpers in the form of `unionToAnyX` are generated. Inside them, there is a switch of type:
```typescript
switch(AnyX[type]) {
case 'NONE': return null;
...
}
```

Where `type` is the actual enum with variants defined as integers starting from 0. So, why not switching directly on `type`, like:
```typescript
switch(type) {
case '0' /* NONE */: return null;
...
}
```

as the code is generated anyway?

Moreover, it might make sense to use `const enum`s for performance reasons, if the added constraints on the API are acceptable.