refactor: make all event emitters readonly

[rafaelss95]

Basically the same idea of PR #9586, which fixed #9582, but unfortunately didn't add a TSLint rule to keep these changes consistent.

Now, I'm adding Codelyzer's prefer-output-readonly rule to the tslint.json :).

[crisbeto]

This feels redundant IMO. It makes us write more code while enforcing a rule that hasn't been an issue until now.

[rafaelss95]

This feels redundant IMO. It makes us write more code while enforcing a rule that hasn't been an issue until now.

Hmm... I thought it was an issue since #9582 was created explicitly to do this. The problem is that in the PR abovementioned I didn't add a rule to keep it consistent. That said, should I close this then?

[crisbeto]

I think that @jelbourn can say whether that issue is still relevant since it's very old.

[jelbourn]

I do think we should still do this. It may be a bit of extra effort, but it is more correct.

[mmalerba]

LGTM

[rafaelss95]

@mmalerba, could you please suggest a commit message to make lint happy? :)

[crisbeto]

You have to add a scope to it, e.g. refactor(<scope>): make all event emitters readonly. These are the possible scopes: https://github.com/angular/components/blob/master/.ng-dev/commit-message.ts#L10

[rafaelss95]

@crisbeto Thanks for the link! I looked at it and I couldn't find a suitable one, as the changes are being applied to several places at the same time.

[mmalerba]

Yeah our commit message format doesn't work well for changes like this. You can just pick one, material/core maybe

[annieyw]

@rafaelss95 please rebase/fixup message when you get the chance

[rafaelss95]

@mmalerba @annieyw sorry for the delay, I rebased and fixed up the commit message.

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