Dart also suffered from external politics: a chrome-first Dartium VM stank of ActiveX to most people. IIRC Microsoft had discontonued its silverlight plugin for lack of interest/support. The general attitude began to shift towards just JS/HTML5 tooling in the webdev community.

And in addition to not having full support from Google, there was not and probably would never have been buy-in from Mozilla or Microsoft. Also the language IMHO didn't offer much more than JavaScript, and 6to5 -> Babel was gaining traction at the same time. It wasn't a good recipe for success.