Improve `reduceComputed` array check assertion.

In #4078 the problem is that `reduceComputed` cannot be used against 
non-extended native arrays as we cannot `addArrayObserver`.  `reduceComputed` 
used to inform users of this, but no longer does so since support for
non-array dependent keys.  This commit corrects this grievous error.

Fix #4078

cc @alexspeller