mixinFunction polymer export default Polymer . dedupingMixin ( ( base ) => { mixinClass polymer class CSSReflectionMixin extends base { param prop param fallback returns readCSSCustomProperty ( prop , fallback ? ) { let value ; if ( 'ShadyCSS' in window ) { value = ( window as any ) . ShadyCSS . getComputedStyleValue ( this , prop ) ; } else { value = getComputedStyle ( this as any , prop ) ; } return value || fallback ; } } return CSSReflectionMixin ; } ) ;