Polymer used to have it's own way of declaring web components to get rid of lots of boilerplate. With v1 of webcomponents, the syntax is similar to React and other class based component frameworks and since v1 is already supported in multiple browsers, they removed the abstractions and are now using native webcomponents' syntax.

Possibly relates to how the syntax will be very similar to how to write native custom elements with ES6 as shown in the specifications here: https://w3c.github.io/webcomponents/spec/custom/#custom-elem...