Polyfill: @polyfill @host rules are broken in an extended element

[morethanreal]

For example, http://jsbin.com/aSotEYA/2/edit

For x-bar,

@polyfill @host > *

becomes

x-bar x-foo > *, [is=x-bar] x-foo > *

[sorvell]

Fixed by googlearchive/platform@6db75b5