(Not speaking for the Dart Team--just my 0.02USD on the topic.)
AS3 shows its design heritage as a dialect of JS; it falls prey to the
same difficulties JS experiences with large-scale development. Dart
wants to move beyond the operational theater of JS into an environment
where multiple applications don't step on each other. It wants to be
a language and environment in which medium- to large-scale development
is easier than with JS. AS3 steps in that direction, but which way is
it facing? To me it looks like it's walking in backwards.
AS3 was designed to drive Flash apps. With HTML5 browsers in the
marketplace Flash is dead on its feet right now, but hasn't yet had
the sense to lie down and stop breathing. (Team Flash will no doubt
leap in to disagree. Leap away.)
There's no reason that Dart shouldn't be attempted. (After all, you
could easily ask why not just crank up the jam on a new version of
VBScript. There's any number of languages that could fill in the
blank.) The Dart team suggests that JS, and JS-derived languages, are
too broken from a base design standpoint to be repaired; at least, if
you'd have to wait 3-4 years for the browser world to encompass JS
Elite, it would be just as efficacious to design and implement a
language from the ground up that takes into account all the problems
encountered in previous attempts.
We already had C--why did we need C++?
We already had C++--why did we need Java?
We already had Java--why did we need C#?
We already had COBOL....