The lastIndex property must be written with the semantics of a strict [[Set]],
so an exception must be thrown when the attributes don't allow writing.  We used
to ignore the attributes.

R=littledan@chromium.org, yangguo@chromium.org
BUG=v8:5138

Review-Url: https://codereview.chromium.org/2109593002
Cr-Commit-Position: refs/heads/master@{#37369}