fix: properly remove event listeners for memory leak prevention

[ludinov]

This PR is just to show buggy code, and possible fix related to #5270
The final code should be at least re-factored a bit and correspondent tests should be added.
I'll not be able to do right PR for now...

[mary-poppins]

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[IgorMinar]

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[ludinov]

1. CLA signed, but I don't expect this draft code will be accepted.
2. Failure examples updated - http://jsfiddle.net/VmTsD/2/
3. Looks like eventHandler.elem = element; doesn't cause leaks (at least in Chrome)..

[GabrielDelepine]

+1

[btford]

After some investigation, this isn't actually an issue. As the node is cleaned up, the handler function is removed. Removing it when there are no events potentially creates unnecessary GC pressure if you add/remove listeners.

[ludinov]

Maybe it's not a critical issue for modern js engines in terms of memory. But is it the reason to don't fix wrong code? Now lines # 218 & # 224 actually do nothing. I'm not sure about GC pressure, but jQuery for example (not lite) remove listeners properly. And now it is hard to find the reall app problem when you see a lot of unreleased garbage from angular core.