Fix material tree selection change detection

[yuan-kuan]

This an attempt to fix #478

I suspect this is a regression caused by this commit: https://github.com/dart-lang/angular_components/commit/259039e1ee9ee540c581684e7b0cdae345dfa101. I found to this commit by going through the history to look for anything that affects change detection. And this is exactly something to do with that. Reverting these changes (removing the new lines added) will fix this issue.

Of course, I am not suggesting undoing these changes. I understand there was a warning about ChangeDetection.OnPush during the building of Angular web application since the angular 6.0.0 upgrade.

So, I thought the correct way is to make MaterialTreeNode to be aware of the selection changes, and mark itself for check.

I have tried to do this selection change detection in the parent component MaterialTreeComponent but failed to find a way to do it from there. So I resorted to do this detection in each tree node.

I have see this fix in my own reproducing project: https://github.com/yuan-kuan/angulardart-broken-tree. And also the Angular Component Example project (I forked and cloned it and tested locally in my machine.)

[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[yuan-kuan]

@googlebot I signed it

[kevmoo]

Already fixed in https://github.com/dart-lang/angular_components/commit/424808415361305c903cf89f4d119020df65a0c2