[Js] allow __instanceof__ (fixed issue #1846)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6692 f16182fa-f095-11de-8f43-4547254af6c6
fixed toString override in IE8 (fixed issue #1850)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6693 f16182fa-f095-11de-8f43-4547254af6c6