"Ubiquitous" ActiveX without Windows is a pipe dream, not a useful premise. Kind of like assuming world peace first, then resolving a lesser conflict.
With ActiveX, the interfaces that might be used are too many, and their implementations too large and complicated, to have bug for bug compatibility. Even having source code would not be enough. All-paths testing would be needed.
Someone could (and a few companies did, for COM on Unix) tediously reverse-engineer a subset of COM interfaces and components implementing them. No one ever pulled off anywhere near a full Windows workalike.
The same threat arises with delayed-open-source controlled by a single proprietor. You can port and fork such code, but you can't depend on the single proprietor, especially if it's a hostile competitor. You'll have to co-maintain if you don't make a long-term fork of your own.
Source != spec. An implementation will over-specify in its source code and API. Abstractions leak. Standards can and do turn down specificity by dropping to prose or more formal means, without overspecifying.
"The risks of Google behaving like a Microsoft-style monopolist here are low because (a) they have no monopoly in browsers, VMs, or OSes to abuse,"
Look closer: Google is a search monopoly in many locales, and they are an emerging duopoly member (the larger share than Apple) on mobile, whose growth predicts it dominating desktop.
"(b) their culture is the most hacker-driven of any large company"
That was true a few years ago. It is much less so now. Larry has cut back on the thousand flowers, and focused on a few strategic bets: Android, Chrome, Google+, Search.
"(c) their interests are aligned with what is good for the web,"
So you say (and perhaps Google people say this, but I know some who candidly admit it just ain't so any longer).
Why do you believe this? As a public company, Google has to show quarterly good results, not just great profit margins but bubbly growth, to keep its stock appreciating, to retain and recruit (see the Facebook defection problem of last year). This is a big distortion on a pure open web mission.
"and (d) the industry has changed since the bad old days."
And human nature has changed since the 20th century, or the French Revolution, or the dark ages? Yeah, right.
You're much closer to the details of all this than I am, and it's possible I've got them wrong. But I can't agree that there's nothing that can make Dart a positive long-term contribution to the web, regardless of how good it turns out to be. I'm willing to be proven wrong about that, but only by the actual outcome. In the meantime, I'm excited - purely because of the track record of the creators. If and when my hopes are dashed I'll come back and post a mea culpa.