When defining an indexed property on an Array object, the object's
length property should (perhaps) be updated.  This was done for any
property for which

  ToUInt32(name) == ToNumber(name)

was true, meaning any property name that, when converted to a number,
was an integer in the range [0, 2^32).  The detection should be more
strict; an indexed property is one for which

  ToString(ToUInt32(name)) == name

is true only.

Review URL: https://codereview.chromium.org/13914003

Patch from Jens Lindström <jl@opera.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14242 ce2b1a6d-e550-0410-aec6-3dcde31c8c00