Warnings in dartium when using some paper/core ele

Question

I'm trying to use paper-tabs, paper-tab and core-pages elements but dartium shows a bunch of warnings like:

(from html5lib) Unexpected non-space characters. Expected DOCTYPE. 
package:paper_elements/paper_tabs.dart:1:1

(from html5lib) Unexpected character after attribute value. 
package:paper_elements/paper_tabs.dart:38:9

And then:

custom element with name "paper-tabs" not found. 

As you may guess these elements doesn't work. Other elements like core-overlay, core-field, paper-button seems to work fine. I'm using dart 1.9.0-dev.3.0, core_elements 0.6.0+5, paper_elements 0.6.1 and polymer 0.15.5+4. What may be the cause of such behaviour?

Answer 1

This is caused by some html imports pointing to dart files instead of html files inside of foo_element.html (code here). Swapping those imports to point to html files removes all warnings.