arrows inversed in mat-tree RTL

[amitport]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

arrows should point left

What is the current behavior?

they're pointing right

What are the steps to reproduce?

see tree in demo-app, RTL mode

What is the use-case or motivation for changing an existing behavior?

visual

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

material 6.0.0-rc.12

Is there anything else we should know?

this was pointed out by @odahcam in #3175 (comment)
I'm just opening this issue so it won't get lost in the comments thread and hopefully resolved before v6

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