I vote for this too. It would be nice to use the modern features of ECMAScript in our test code.
As for the complaint about not being able to use prototype functions across Script Units, we have come up with an ugly hack to get around that limitation. Basically, at the top of every script unit, we run the equivalent of this:
for(var i in SystemUtilities.Array.prototype)
{
if(Array.prototype == undefined)
Array.prototype = SystemUtilities.Array.prototype;
}
SystemUtilities is the name of the Scirpt Unit that the prototype functions are written in. We don't actually put these lines in every file, but eval the output of a function which is a string to this effect.