fix(memory leak): memory leaking in UniqueSelectionDispatcher

[Eddman]

Listeners are only registered inside of UniqueSelectionDispatcher. But there were never removed causing in my case a small memory leak when using MdButtonToggle.

In this fix UniqueSelectionDispatcher.listen returns a function that removes the listener once called.

[Eddman]

Please merge ASAP: it is causing huge mem-leaks in my application so it makes the Chrome tab to crash in few minutes...

In combination with #5144 I'm almost unable to use the library. We are managing a big data set that is leaking because of these...

[jelbourn]

Thanks for the PR! Just a few comments

[jelbourn]

The return type is () => void

[jelbourn]

Call this _removeUniqueSelectionListener, type is () => void and initialize it to () => {}:

private _removeUniqueSelectionListener: () => void = () => {};

Then you don't have to check if it is defined in the ngOnDestroy

(same change for other components as well)

@devversion this is potentially a good case for a mixin in a follow-up PR

[jelbourn]

Prefer line-breaking at the higher syntactic level, e.g.

this._removeUniqueSelectionListener =
    _buttonToggleDispatcher.listen((id: string, name: string) => {
      if (id != this.id && name == this.name) {
        this.checked = false;
      }
    });

[jelbourn]

This also needs a unit test. It should set up a listener, then deregister and assert that listener isn't called again on notify

[Eddman]

Done. See unique-selection-dispatcher.spec.ts.

[Eddman]

@jelbourn All comments are implemented.

[jelbourn]

LGTM

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