With the exception of numbers we should be conformant (and if we aren't please file a bug). A fully compliant dart2js would be theoretically nice, but practically mostly useless (because of its speed).

The Dart language and its spec are independent of dart2js (which is just an implementation striving to implement the spec). Clearly there is a huge connection between both (same team, ...), but in the end we decided to accept the non-conformance for the benefit of a better language. On the server Dart developers can already enjoy the more complete number-hierarchy, and, even if it takes time, I still have the hope that client-side we will eventually be able to enjoy Dart VMs everywhere. (Call me "optimist" ;)

Wrt debugging: the VM has a mode where it detects incompatibilities. For example when a number exceeds the 53bits of JS. This should reduce the difficulties in developing cross-platform tremendously.