There are some things in Dart that would be nice to have in Javascript, but I don't see why Google wouldn't just try to work them into future ECMAScript revisions. With the exception of banning eval and run-time modification of prototypes, I don't see anything that's completely incompatible with a future revision of Javascript.

I'd even bet that the ES standards committee will eventually give in on the "arrow" lambda style. ES5 has the "function(x) x * x" shorthand, but with C# and Java both settling on "(x) => x * x", I can see it happening in time.