Fixed evaluation of computed property names with function expressions.

Fixed evaluation of computed property names with function expressions.

Previously, while evaluating a property name expression with a function
expression as the right side, the evaluation modified the value used to
compute the property name in-place.  The in-place modification changes
values not intended to be changed.

The issue was introduced in 74d30c2d70f3 (0.7.8).

This fixes #640 issue on Github.