paper-dialog opens into wrong parent if parent doesn't have an ID

If you create two (or more) `paper-dialog`s and pass a different `parent` to each in the form of a jQuery object, but neither parent has an `id`, the `destinationId` computed property in `paper-dialog` will set both parents' `id` to `null-parent`. This results in both dialogs opening into the first parent.

This happens because the `destinationId` computed property attempts to append `-parent` to the dialog's `elementId` to form an `id` for the parent element - but `paper-dialog` is a tagless component, and therefore does not (and cannot) have an `elementId`.

One workaround for this issue is to ensure that every element which will be used as a `paper-dialog`'s `parent` has a (unique) `id`, as that will prevent the computed property from setting it to `null-parent`.