Putting IE event code inside of (almost) any closure causes leaks

[sjmiles]

The IE event connect code goes to great links to avoid attaching anything but primitive values to event handlers to avoid confusing the IE garbage collector. It's easy to accidentally attach references to handlers via closures (there is a warnung in the code to that effect.)

Sometimes Dojo itself gets put in a closure which causes leaks (in particular, the new scope mapping stuff uses this technique).

Fix: replacing the inline dispatcher function with a function created via new keyword provides more robust closure prevention.

[sjmiles]

(In [12848]) Fixes #6050, make sure to clear _stealthKeydownHandle, so key listener can be reconnected later.
Fixes #6063, use new Function() to generate "pure" (closes over nothing) functions. !strict (return value complaints)