HTML5 specifies "type" attribute for "script" tags http://www.whatwg.org/specs/web-apps/current-work/multipage/... The type attribute gives the language of the script or format of the data. I see no reason why some WebKit developers want to restrict what languages people can add by technical means. If there's a political reason to restrict it, they probably should discuss it within W3C and remove "type" attribute if…
It would also saddle them with an extra maintenance & support burden for the foreseeable future, to the strategic benefit of nobody but Google.
I think the original conversation was hampered by a miscommunication - it was never made clear whether it was just about hacking on a custom version of WebKit, or about adding Dart support to WebKit based browsers.