super.prop assignment can silently overwrite non-writable properties

[Caitlin Potter]

It makes perfect sense for Object.defineProperty, but maybe not so much sense for PutValue(). One idea was to just add an `return false if existingDescriptor.[[Writable]] is false.` Before receiver.[[DefineOwnProperty]]()`. 

> On Apr 20, 2015, at 8:17 PM, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
> 
> 
>> On Apr 20, 2015, at 12:42 PM, Caitlin Potter wrote:
>> 
>> Oh — he’s right, ValidateAndApplyPropertyDescriptor won’t throw in the example case, because the old descriptor is configurable. That’s kind of weird.
> 
> It is kind of weird, but that was what TC39 decided on back when ES5 was being developed.  The logic was that if a property is configurable then it is possible to change all of its attributes  by performing a [[DefineOwnProperty]] with a complete property description.   Because of that possibility, all changes made via a partial property descriptor are also accepted.  In other words:
> 
> var o = Object.create(null, {x:{value: 0, writable: false, enumerable: true, configurable:true}});
> Object.defineProperty(o,' x', {value:2});
> console.log(o.x); //2
> 
> The define property above is allowed because it could have been replaced with the sequence :
> Object.defineProperty(o,'x', {writable: true});
> Object.defineProperty(o,'x', {value: 2, writable: false});
> 
> or even by:
> delete o.x;
> Object.defineProperty(o,'x', {value: 2, writable: false, enumerable: true, configurable: true};)
> 
> hence, we might as well accept the single line version.
> 
> In retrospect, perhaps not such a good idea. 
> 
> Allen
> 
> 
>