Allen Wirfs-Brock (2014-06-12T15:26:23.000Z)
domenic at domenicdenicola.com (2014-06-20T19:31:52.027Z)
There's a lot of subtle stuff going on with these compatibility cases and addressing them was something we needed to do to get consensus around supporting subclassing of Array. I don't think we want to mess them up simply to avoid spec. level use of IsConstructor. I really don't expect anybody to seriously try to self-host the compatibility aspects of those methods and there is little point in trying to do a ES5 level shim that includes that functionality because other aspects of of subclassing just won't work in ES5.