[ShadyCSS] :host() selector containing multiple elements extremely slow when first element has a long name

Complete example

```

:host(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, a) {
}

class ThemeBug extends Polymer.Element {
static get is() { return 'theme-bug'; }
}

window.customElements.define(ThemeBug.is, ThemeBug);

```

When there are two elements within `:host()`, and the length of the first gets to around 20-30 characters, the page loading time gets slow to the point that e.g. Firefox freezes completely.

If the order of the long and short names are reversed, this is not an issue.