Figuring out the behavior of WindowProxy in the face of non-configurable properties

[Brendan Eich]

Mark S. Miller wrote:
> The reason why the intent is unwarranted is that the descriptor omits 
> "configurable:" rather than explicitly saying "configurable: true". If 
> the owner object already has a configurable own property of the same 
> name, then a defineProperty where the "configurable:" is omitted 
> defines an own property preserving the configurability of the original 
> own property.

Wild, and genius. How many more narrow escapes can we make and keep both 
web compat and integrity? :-P

Is there any downside? What is the bad case that observably changes 
behavior, if any (not involving proxies)? I'm too tired to search the 
state space right now, throwing this out as a challenge.

/be