Add a platform object [[Set]] internal method so we can have prototype setters show through even if an object has a named property of the same name and can have named creators work correctly on [OverrideBuiltins] objects even if thre is a property with the same name on the prototype.

This fixes <https://www.w3.org/Bugs/Public/show_bug.cgi?id=25025> and
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=26521>.

Author: bzbarsky