mat-primary being applied to flat button by default

[brackio]

Bug, feature request, or proposal:

incorrect class applied.

What is the expected behavior?

.mat-primary should only be apply is the color attr is set. For eg, color="primary"

What is the current behavior?

.mat-primary is being applied to Some button by default.

What are the steps to reproduce?

This bug is also visible on the buttons example on the docs site.

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

v6.4.1

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}