[ShadyCSS] Semicolons in mixins are not supported

[43081j]

Pretty much #48 again.

The PR did not fix the issue entirely, it seems mixins are still broken because of this:
http://jsbin.com/pekebeyezi/edit?html,output

Any mixin like such:

--foo: {
  background: url('data:image/png;base64,blah');
}

Will mistakingly terminate the mixin after the semicolon.

[tomalec]

Seems like a regression of webcomponents/shadycss#48

[tomalec]

Or like similar issue but for MIXIN_MATCH

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.