Icons are not mirrored on RTL

[philip-firstorder]

Bug, feature request, or proposal:

Icons are not mirrored on RTL like in the guidelines.

What is the expected behavior?

To mirror all the icons on RTL except the ones described in chapter "When not to mirror" (refresh, history, keyboard, headset, search, local_cafe, slashed icons, media playback icons, ...)

I think applying a transform: scale(-1, 1); will do the job, making sure to skip the excluded icons.

What is the current behavior?

Icons are not mirrored at all.

What are the steps to reproduce?

Press RTL
https://tina-material-tree.firebaseapp.com/button

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

To follow the guidelines on mirroring the icons on RTL.

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

"@angular/material": "^5.2.1",

Is there anything else we should know?

I found a list of icons that can be mirrored here: http://google.github.io/material-design-icons/#icons-in-rtl

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