Basic raised button text color is wrong in dark theme

[QimatLuo]

Bug, feature request, or proposal:

<button mat-raised-button>Button</button>

What is the expected behavior?

It should looks like example https://material.angular.io/components/button/examples

What is the current behavior?

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-2jzs4u?file=styles.scss
I only change theme to pink-bluegrey.css

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

Just like a bug. Because I follow example code but get different result with example UI.

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

Angular Material 5.1.0

[devversion]

Related to #4614 and dcad946