First, it can't without careful separation of its incompatible breaks with JS, from whatever optional type system is standardizable in future JS. The two are intertwined in Dart right now. JS is not going to break compat or add opt-in version selection to runtime-incompatible modes. That died with ES4, buried by 1JS.
Second, 2030 or even 2020 is way too far out, given the interest in TC39 right now from Google, Facebook, Microsoft, and other members. And because it took the long-odds bet, Dart is now clearly less likely than TypeScript => StrongScript to have any say on future JS optional types (AKA the revenge of ES4). You write
"I also think it gives a lot of real world usage data helpful for getting optional types into ES. (You seem to disagree here.)"
I'm not disagreeing just as a speculative opinion about 2020. I'm telling you that Dart has missed the optional types boat now, in TC39 -- the next meeting in less than two weeks. Dart and any real-world usage data from it are nowhere, while StrongScript is "on" and should generate V8-based data this year.
On the laundry list of "optional types / completion / doc popups...": most of this has been done without Dart in full or even Dart's optional types, including in the Closure compiler. (See DoctorJS for early higher-order control flow analysis that could do completion without making programmers declare types all over.)
Even the claim that a new VM is needed to do the complete laundry list is suspect at this point. Yeah, it's nice to have a clean slate, or one starting from StrongTalk's slate, as Dart did. But if only Google can afford it, it diverges incompatibly up front, and it can only partially re-converge far in the future, then it is very likely a "miss".
Life is not just a series of random Homer Simpson events, with everything about possible futures a matter of opinion. People bet on futures profitably, beating fair coin tosses. Alan Kay was right, the best way to predict is to invent.
So the smart money is not on Dart designing future optional types for JS. This could change but I doubt it, since Dart broke too much compatibility, unnecessarily for the purposes of making types optional. No one is disentangling and minimizing a proposal for ES7 or ES8 (2016 or 2017), while StrongScript is in pole position. This is much more like how standards evolution should work than the silly Dash memo's model.