Menu: MenuClose - Subscriptions are not cleaned up

[bene-starzengruber]

Bug, feature request, or proposal:

If the MatMenuTrigger element is destroyed, the menuClosedSubscription on the MatMenu is not cleaned up which leads to a memory leak as the number of observers is increasing over time.

What is the expected behavior?

The MatMenuTrigger should unsubscribe it's _menuCloseSubscription on the MatMenu in the cleanUpSubscription method

https://github.com/angular/material2/blob/1fb1c5550e426906cef03357b25971aa486d8c68/src/lib/menu/menu-trigger.ts#L453-L456

What is the current behavior?

The _menuCloseSubscription is not unsubscribed in ngOnDestroy() -> _cleanUpSubscriptions(), so the old observer is still bound to the menu which prevents the GC to release the old Observer.

What are the steps to reproduce?

Everytime that the trigger is destroyed and created again, an additional observer is created
https://stackblitz.com/edit/angular-svnkjn?file=src%2Fapp%2Fapp.component.ts

What is the use-case or motivation for changing an existing behavior?

Can lead to massive memory leaks in some use cases

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular 7.0.3, Windows 10, Typescript 3.1.1

Is there anything else we should know?

It seems like the bug was introduced with this commit: fc5f5d3

This topic came up during commit discussions:

But in the case that only the MatTrigger is destroyed and the Panel still exists - the close stream is not completed

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