Use Object.defineProperty() rather than deprecated __setProperty__() and getProperty__()
Use Object.defineProperty() rather than deprecated __setProperty__() and getProperty__()

This fixes https://github.com/web-animations/web-animations-js/issues/106

dstockwell/web-animations-js,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,dstockwell/web-animations-js,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,birtles/web-animations-js,birtles/web-animations-js