Proposal - Allow 'this' to be used as type

[NN---]

When 'this' is used as type it is replaced with the following interface or class.

class A {
 f(a : this) {} // <=> f(a : A)
 g(x : any) { 
   if (x instanceof this) { } // <=> x instanceof A
}

interface B {
  x : this; // <=> x : B
}

[DanielRosenwasser]

This is actually a duplicate of #285 (and #1435).

For the record, this-type as an input type is not type safe, as I'll elaborate on in that issue.

[NN---]

Why not type safe?
If this always resolves to the static type of the class it is purely type safe.

I understand the other proposals is to dynamically bind this to the real type beneath, I don't propose it.
Maybe 'typeof this' makes it more clear.