use selective mixin on platforms without __proto__ (namely IE) for efficiency and to support wrapper environments (aka ShadowDOM polyfill) where the DOM prototypes may be custom
in mixin platforms (no __proto__ support) the effective base prototype is not part of the real prototype chain but instead is hinted as  __proto__ (by CustomElements.js)
'enumerable' no longer necessary under improved mixin strategy (customMixin in CustomElements.js)