Might be more accurate to say that Javascript was becoming ActionScript - ActionScript 3 was based on a draft standard for EcmaScript 4, which at the time of AS3's creation looked set to become JavaScript 2.0, but that didn't end up panning out.
It's my understanding that Adobe has every intention of remaining a superset of EcmaScript. If ES breaks AS3 compatibility in the future, it's safe to expect that Adobe will keep pace.
FWIW, for much of its documentation, Adobe refers to the EcmaScript specification. In fact, Flash instructions map 1:1 to EcmaScript specs, including the type conversion and promotion algorithms.
