Mat-tree is broken on examples

[dinohorvat]

Bug, feature request, or proposal:

mat-tree can be easily broken. Tested it on your demo
https://material.angular.io/components/tree/examples

What is the current behavior?

What are the steps to reproduce?

1. Try opening parent node, and then open child node as well.
2. Close parent node.
3. Repeat

You will end up creating multiple child nodes, every time you do this procedure.

[tinayuangao]

Fixed by #11728

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