[draggable] track draggable item scopes

[spalger]

Fixes #9151

The bug described in #9151 is caused by the removal of the angular debug info in #7929. A second quick-fix option is to track the element scopes inside the draggable container with a WeakMap().

#9152 is an alternative solution

[stacey-gammon]

Interesting. Do you know why removing angular debug info broke this? I'm curious why it worked with the debug info. Perhaps the debug code somehow stops the elements scope from being garbage collected? Makes me nervous that there are similar bugs, as yet unfound, that suffer the same problem. That PR is about a month old but if it took 30 days to realize we can't create any visualizations in non-dev mode.... there might be more issues lurking.

What are your thoughts @spalger?

[spalger]

This code was previously dependent on the $.scope() function which uses the debug info attached to elements to lookup the scope for elements in angular's internal store. This function just returns nothing when debug info is disabled (unfortunately) which is why this code is now tracking the scopes for child directives in a WeakMap.

I totally agree with the notion that this could indicate other undiscovered issues. I've verified that there isn't any other code (except tests) using the .scope() function and I think the rest can be left to smoke testing/qa

[kobelb]

Are we polyfilling WeakMap? Support seems to be rather non-existent for IE 11. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap

[spalger]

Yep, we use the core-js implementation by virtue of the babel-polyfill

[kobelb]

LGTM

[stacey-gammon]

Ahah, I see now in the angular documentation about scope, though they aren't very clear that you shouldn't use it outside of debugging though:

Scopes are attached to the DOM as $scope data property, and can be retrieved for debugging purposes. (It is unlikely that one would need to retrieve scopes in this way inside the application.) 

Thanks for the explanation, lgtm!

[spalger]

Backported to 5.1 in #9250