refactor(button): use color binding for fab button color

[devversion]

No longer creates extra CSS selectors to set the default color for round buttons to accent.

@tinayuangao @jelbourn This needs some confirmation in regards to the _hasAttributeWithPrefix method. The isBrowser check in that method doesn't seem to be a good solution for Universal?

Another possible approach

To be sure that this works in Univeral we can also do it the following way:

[jelbourn]

@devversion change looks good, not sure what you mean by the _hasAttributeWithPrefix / isBrowser stuff, though. That isBrowser check is specific there so that Universal does work.

[devversion]

@jelbourn I thought that if someone renders a fab button on the server the _hasAttributeWithPrefix will always return false (due to the isBrowser check) and therefore the fab will not get the accent color by default.

[jelbourn]

Oh yeah, that would be a problem. When I added the isBrowser check, the only thing that this was used for only dealt with styles for user interaction.

[devversion]

Yeah with that it was totally reasonable to just skip in non-browser environments.

What do you think about the other approach I added to the PR description?

[jelbourn]

I think that alternate approach would be necessary

[devversion]

@jelbourn Done. This should be the "Angular"-way of ensuring that it works with Universal.

[tinayuangao]

LGTM

[andrewseguin]

Here and in MdMiniFab, looks like we want to use forwardRef since those components are defined further down the file. This caused problems internally, surprised our tests didn't have trouble with them.

[devversion]

@andrewseguin Done.

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