Make 'new.target' emit more precautions

I noticed that the polyfill for new.target ...

class Foo {
    constructor() {
        if (new.target)
            alert("Good.");
        else
            alert("Bad!");
    }
}

... outputs to this:

var Foo = (function () {
    function Foo() {
        var _newTarget = this.constructor;
        if (_newTarget)
            alert("Good.");
        else
            alert("Bad!");
    }
    return Foo;
}());

That seems dangerous to me, since this is always a true condition for older browsers that don't yet support new.target. Does it not make more sense to create an output such as this?:

var Foo = (function () {
    function Foo() {
        var _newTarget = this && this.constructor !== Window ? this.constructor : void 0;
                         /* (have to check 'this' also, in case of strict mode) */
        if (_newTarget)
            alert("Good.");
        else
            alert("Bad!");
    }
    return Foo;
}());

The following code fails to work as expected in Chrome v56.0.2924.87 (output is "Good." in call cases):

class $Foo {
    constructor() {
        if (new.target)
            alert("Good.");
        else
            alert("Bad!");
    }
}

type FooConstructor = typeof $Foo;

interface CallableFoo extends FooConstructor { (): $Foo; }

new $Foo(); // ok
var Foo: CallableFoo = <any>Foo;
Foo(); // ok? :/

Inizia con il repro collegato di TypeScript Playground e confronta il JavaScript emesso per new $Foo() e Foo(). Segui il lowering di new.target nel compilatore; il lavoro è concluso quando l'output generato preserva i risultati diversi per le chiamate al costruttore e le chiamate ordinarie, senza trattarle entrambe come truthy.