Tag element templates with Polymer.html

[justinfagnani]

Fixes #239

[justinfagnani]

Er, don't review yet... I forgot some things

[justinfagnani]

Ok, updated some unit tests to make sure that the added Polymer.html references were indeed being converted properly to JS imports. All seems well there.

I still haven't regenerated the fixtures due to what looks like bad fixture generation with my repo. @FredKSchott could you try to see what happens with fixture generation with a clean checkout on your end?

[FredKSchott]

(Edit, re the above: we talked in person)
LMK when this is ready for another review

[justinfagnani]

@FredKSchott rebase, removed workarounds for not having html-tag.html, and ready for review again.

[FredKSchott]

Looks great!

I was just thinking of another way to tackle these fixture updates: a lot of the changes are from the fact that source has been updated. we could first regenerate fixtures from the last-used version. Then regenerate from master, and that diff would have much much much less noise.

Something to keep in mind for next time.

[justinfagnani]

I was thinking of that yesterday, but had already generated these commits and the change was relatively small. Good idea though.