add mixin support

fixes https://github.com/dart-lang/custom-element-apigen/issues/15
when I think of polymer object, I mainly think of the element declaration `Polymer(name, {})`, but not really of the mixins. Maybe rename it? One idea following the pattern we have with the other AST nodes would be something like `Class` (similar to the naming we use for `Property` or `Method`). Another option would be a name to match `NamedEntry` and `TypedEntry`, maybe `DeclaredEntry`? The latter sounds a bit weird to me too, so my current vote is for `Class`. WDYT?