Do not turn source array elements into writable if doing Array.slice.

Do not turn source array elements into writable if doing Array.slice.

Array.slice doesn't mutate original array, so it's fine with read only data.
Plus nuke unnecessary cast.

Committed: http://code.google.com/p/v8/source/detail?r=6113

[antonm, 2010-12-22 11:50:38]

Vitaly,

may you have a look?

[Vitaly Repeshko, 2010-12-22 15:19:18]

LGTM