build: add root to the sourcemap so it won't use absolute pathing

[hansl]

Fixes #541, I think.

Need to chat to @jelbourn to try to see if that fixes the release itself, not jsut local.

[hansl]

This requires the latest release of the CLI to get angular/angular-cli#1028.

[hansl]

Also fix prod builds, which actually don't make sense for material, but 🤷

[jelbourn]

I'm going to merge this since it's better, but I think the sourceMaps still aren't exactly right.

[danrasmuson]

Thanks guys. We hit this yesterday.

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