Don't emit __extends if one is already in scope

[RyanCavanaugh]

Addresses #1350 and probably some other suggestions as well.

This changes the "do we need to emit __extends" logic to see if a symbol named __extends is already in visible scope at the site of the class declaration.

[mhegazy]

would this work:

module M {
  export var __extends = () =>{};

  class A { }
  class B extends A { }
}

[RyanCavanaugh]

Good point. Should we have the emitter handle this, or just say that only a global __extends is valid?

[mhegazy]

I can go either ways. limiting it to global __extends seems simple to implement and handles common scenarios.

[CyrusNajmabadi]

please break this into multiple lines. Or provide a helper function for it.