function elementHasBuiltCss(element) {
  return getCssBuild(element) !== '';
}
Check if the given element, either a <template> or <style>, has been processed
by polymer-css-build.

If so, then we can make a number of optimizations:
- polymer-css-build will decompose mixins into individual CSS Custom Properties,
so the ApplyShim can be skipped entirely.
- Under native ShadowDOM, the style text can just be copied into each instance
without modification
- If the build is "shady" and ShadyDOM is in use, the styling does not need
scoping beyond the shimming of CSS Custom Properties

@param {!HTMLElement} element
@return {boolean}