`resolveVariableReference` enforces variables names to be own properties

[caridy]

This seems to be very restrictive, and instead, Reflect.has() is probably better. In our case, the args structure provided to messages might be optimized in various ways (very likely some immutable structure) that does not necessary flatten all data into an object, and instead it might rely on the __proto__ chain.

[caridy]

Closing since this issue belongs to fluent.js instead.