refactor($rootScope): add warnings when emitting/broadcasting on a destroyed scope

[andersekdahl]

When a scope is destroyed, all event listeners gets unregistered, and the connection to
the parent/child scopes are removed. Because of this, no one is noticed of the event.
$emit and $broadcast now logs a warning when trying to emit or broadcast an event on
a destroyed scope.

Closes #6768

[mary-poppins]

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• Uses the issue template (#6784)

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Thanks again for your help!

[andersekdahl]

Yes, well, it's a bit of a slippery slope in that case. If it's the child of a destroyed scope, and the child scope hasn't yet been destroyed then it can still call listeners on the child scope. So my reasoning for only checking this.$$destroyed is that in that case there's no chance of anyone recieving that event.

[thorn0]

Subscopes of destroyed scopes have $$destroyed set to false, at least in v1.3.6.
See https://github.com/angular/angular.js/blob/master/src/ng/rootScope.js#L229

[lgalfaso]

@thorn0 did you mean: set to true?

[thorn0]

Oops, yes, I did.