1. Dart development (Dash, whatever, and there may be more, including CSS and HTML killers; not sure, rumors swirl) has been ongoing for approximately 2 years -- or more. It didn't start last November.
2. Google members of Ecma TC39 have been working on ES.next (some harder than others, I observe) without being able to show how Dart solves "unfixable" JS problems. Such demonstrations would help either:
2a. steer JS toward fixes if the "unfixable" assertion is false (as seems likely to me; little is unfixable on the web), or else:
2b. abandon doomed fix-the-unfixable attempts and instead work harder on other and fixable problems (e.g. being a better target language for Dart-to-JS compilation).
3. Delayed open-source means other browser vendors and volunteers have a high hill to climb to become committers/reviewers/co-owners, so Google controls the open source. This has happened many times. Competitors are unlikely to join, especially if the code is complex and has deep dependencies on other code (cf. NaCl/Pepper).
BTW, WebKit is an example more than a counter-example. It was Apple-dominated even though early-mostly-open, and now Google has taxed Apple committers/reviewers and is gaining the upper hand.
WebKit was early-open, a fork of KHTML at first, then set up as webkit.org in 2005 patterned after mozilla.org and in the aftermath of a recruit-half-the-Safari-team-to-fork-Firefox attempt by Flock. This history shows more open that closed, and earlier open at that, but mixed up with various intrigues and corporate control agendas.
While the history is not a clean win for any point of view, WebKit is a "commons" of its own. Note how chromium.org has to hold the Google-only extensions that Apple et al. won't take.
4. Standardization of Dart could happen anywhere, but it would be perceived as anywhere from wasteful to hostile for Google to bypass Ecma TC39. Early opening of a draft spec or even just an open-source implementation again could have won friends and influenced people on TC39. Late opening goes the other way.
What actually has happened, from what we already know: late-open.