Fixed Array.prototype.slice() when array is changed while iterating.

Fixed Array.prototype.slice() when array is changed while iterating.

Previously, the flat array may be converted to a slow one as a
side-effect of a custom getter invocation for a proto array object.
The function erroneously assumed that the this array remains flat
while iterating.

The fix is to eliminate the micro-optimization which uses direct
pointers.

The problem is similar to the previous (9578cc729205) commit.

This closes #445 issue on Github.