dillon1000/react

Support destructuring assignment of context variables

Fixes T178003134. Previously we did not check whether values reassigned during a 
destructuring assignment were context variables. This would either miscompile, 
or as of my fix earlier in #2579, would fail validation. Specifically, this 
happened on AssignmentEpression with an object/array pattern lvalue, where the 
pattern contained an identifier that is a context variable. 

This is now fixed: we track whether the outermost assignment is a normal 
assignment or destructuring, and force destructuring to a temporary whenever the 
identifier is a context variable. We apply the same logic to variable 
declarations that are destructuring to a context variable.