Fix goog.array.extend to work with large arrays.

Signed off by: yzhu@yahoo-inc.com (under Corporate CLA for Yahoo, Inc.)
Remove superfluous array check in goog.array.extend

Signed off by: yzhu@yahoo-inc.com (under corporate CLA for Yahoo, Inc)
Merge pull request #356 from diracdeltas/fix/extend

Fix goog.array.extend to work with large arrays.