Further refinement to semantics that I have missed in previous change.
Both Blink and Firefox are permissive with arguments to .set method.
However, when first argument to "set" is a number, all implementations
throw, so that users know that
   a.set(0,27)
does not assign 27 to 0th element of a, not 0 to 27th element of a.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/19210002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15684 ce2b1a6d-e550-0410-aec6-3dcde31c8c00