Allow to call getter/setter logic When subclassing

[mwisnicki]

http://typescript.codeplex.com/workitem/645

At the moment it's not possible to call super getter/setter when overriding properties:

class A {
    private _property: string;
    get property() { return this._property; }
    set property(value: string) { this._property = value }
}

class B extends A {
    set property(value: string) {
        super.property = value + " addition";
    }
}

[RyanCavanaugh]

The problem is that we don't surface the fact that a property is a getter/setter in the type information, and it's impossible to emit code here that would work even if the base class's definition was not a getter/setter. Using 'virtual' properties in JavaScript is generally not a good idea for this reason.

/* MyDeclFile.d.ts */
declare class A {
  x: number; // actually implemented as getter/setter
  y: number; // actually implemented as field
}

/* MyConsumer.ts */
class B {
   get x() { return super.x; } // Could theoretically emit code for this
   get y() { return super.y; } // Cannot work; there is no 'super.y' to refer to
}

[fdecampredon]

It's part of es6 class, so it seems pretty important to me, also generating something that throws an error in case of missing super setter and return undefined in case of missing super getter (like what traceur does https://github.com/google/traceur-compiler/blob/master/src/runtime/classes.js#L43-L60) seems pretty much standard and compliant with what future es6 engine will do.

[RyanCavanaugh]

The point about ES6 is very important. We'll need to figure out what that means.