Refs #18168. dojo/on: adjust feature detection such that it also works in iOS 8 (beta) with a built dojo/on stripping the strict mode

The previous feature detection code used by dojo/on for deciding whether to use event delegation is broken if and only if a built dojo/on stripping the strict mode is used on iOS 8 (beta). This is because, in non-strict mode, the assignment of event delegate's property fails without raising an error. (Note that the unbuilt dojo/on is in strict mode, thus is not affected by this issue, and there are means for a built dojo/on to preserve its strict mode).

The present PR fixes the misbehavior in non-strict mode on iOS 8 (beta), and has been tested successfully in strict and non-strict modes, with iOS 8 beta 5 and other OS versions as well.