Platform-server : TypeError: Cannot read property 'dir' of undefined

[mahmood-sajjadi]

Bug, feature request, or proposal:

BUG

What is the expected behavior?

Material should work with platform-server

What is the current behavior?

TypeError: Cannot read property 'dir' of undefined

What are the steps to reproduce?

Please look the next section for source of issue

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

platform-server

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

material: 2.0.0-beta.7
angular: 4.1.2

Is there anything else we should know?

https://github.com/angular/material2/blob/master/src/lib/core/bidi/directionality.ts#L50
in platform-server: _document.documentElement is undefined so _document.documentElement.dir is rasing an error

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