Apply cssResult to instance of an element

[bennypowers]

Description

I'd like to be able to apply a cssResult to an instance of an element, not to the class. An example use case would be dynamic css with unsafeCSS

// imaginary API
this.applyStyleResult(css`${unsafeCSS(this.generateSomeDynamicStyles())}`);

Slack reference: https://polymer.slack.com/archives/C03PF4L4L/p1552653870035900

[kevinpschaaf]

Just noting that the main issue with this would be polyfilled Shadow DOM support, since ShadyCSS currently does not have good support for generic instance-time styling (outside of the CSS custom properties shim).

[sorvell]

Closing in favor of lit/lit#3010.