Normative: Invoke superclass constructor even if its .prototype is null

[bakkot]

Fixes #699: currently the below new does not invoke A; with this change it does.

function A() { this.x = 1; }
A.prototype = null;

new class extends A {}

As mentioned there, this is a normative change, but it causes the spec to match current behavior on (at least) Chrome and Firefox and so probably isn't breaking.

At the moment it does nothing to resolve the scoping issue mentioned here.

[bakkot]

test262 collateral here, since this is a normative change.

[bterlson]

This seems fine to me (and the semantics I intended when implementing a35f0f3). I'll think about some way to avoid the "scoping" issue. Thanks for the test262 collateral!