Re: [w3c/webcomponents] The is="" attribute is confusing? Maybe we should encourage only ES6 class-based extension. (#509)

I don't think the custom elements API should introduce a new, single-scenario composition / inheritance mechanism (`defineBehavior`, `defineExtension`). If there are features that built-in elements have that can't be emulated by custom elements because they aren't exposed by the platform, we should be working to expose them (@rniwa's suggestion) rather than claiming that they exist because of a separate, element-only method of composition.

Given that the features in question aren't currently exposed in any form (other than as part of the built-in elements), why concede that we need a special way to compose them? Wouldn't it be preferable to expose them in a way that makes them composable in plain JavaScript? Further, if this new, custom-elements-only composition API then isn't necessary for using these features in your custom element, why should it become part of (read: add complexity to) the custom elements API? This seems like a 'solution' looking for a problem.

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