The only documentation is bare-bones examples - 0 docs about conversion between dart<->js objects/collections/closures etc. when crossing JS interfaces. You will agree that this is an area which requires very precise description of semantics - after all we are bridging two dynamic languages with two incompatible object interfaces one on top of the other - reverse engineering the semantics sounds like a nightmare.

From here : https://github.com/dart-lang/js-interop-deprecated I was able to find this : https://docs.google.com/document/d/1X0M7iQ1PraH50353OnjKidgr... which was last updated in 2014 and I'm not sure if it's relevant.