mat-tree expand collapse all functionality

[saidoze]

Proposal:

Is there a possibility to collapse and/or expand all nodes at once?

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

6.0.1

[jelbourn]

Please keep GitHub issues for bug reports / feature requests. Better avenues for troubleshooting / questions are stack overflow, gitter, mailing list, etc.

(this is a canned response)

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