fix(transformer): Throw unimplemented errors in HtmlAdapter.

[jakemac53]

closes #2624

[tbosch]

This needs a little more investigation, as we actually should not call appendChild in this case at all...

[tbosch]

Will look into it

[tbosch]

Ok, double checked, this is correct. I got confused by some debugging that showed that we append children directly under a <template> tag, and not under its content document fragment. However, the html pub package does not have a content document fragment, so this is good.

Thannks @jakemac53 !

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