Investigate why we need `__dataHasAccessor = {partial: true, viewModel: true`

[tomalec]

see discussion at https://github.com/Juicy/imported-template/pull/56#issuecomment-390980670

Steps to reproduce

1. Go to https://github.com/Juicy/imported-template/blob/master/imported-template.html#L172

It hacks Polymer notification protocol to forward partial, viewModel even though we only use model. Removing them seems to breaks some tests https://github.com/Juicy/imported-template/pull/56#issuecomment-390980670

Expected result

imported-template should care only about its own properties.

Actual result

It seems it may cover something on behalf of starcounter-include

//cc @alshakero