When we are creating a type definition file for a class has a private constructor,
we convert it to a combination of an instance of a concrete type with no constructor and an interface that contains the other methods.
When this happens, we must also convert the class's heritage from
`implements` to `extends` since interfaces cannot implement other interfaces
or classes.
Fixes a problem with #2996Closes#3002
This means that we can now run just the d.ts file generation by running:
```bash
gulp docs/typings
```
In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.
Peter Bacon Darwin