See my other reply on XHR. It is orders of magnitude simpler than Dart. Also, even though MS had monopoly power when they added XHR, they did not abuse it on the web. XHR was for OWA. Mozilla and Opera were not coerced by MS-inspired use of XHR on the web. We chose to implement (with some changes, see the wikipedia page) at leisure.
In contrast, Dart usage in Google web properties targeting the native VM in Chrome, per the leaked memo, would pressure other browsers to adopt or reverse-engineer a much more complex non-standard.