Only until there is enough adoption for them to stop caring.

If there's enough adoption, then I'd expect other browser developers either to build their own Dart runtimes, or -- since its free -- use or fork Google's.

And, then, sure, Dart2JS stops being important to Google because its unimportant to how Dart code performs on the web. But its unimportant to Google because its unimportant to anyone, since it won't matter to users, either.

If there is that much adoption then the onus will be on Mozilla, Apple and Microsoft etc to step up and accept the will of web developers. Edit: and push for standardization.

Then get left in the dust as Chrome takes advantage of undocumented features.

We've seen that game before, played by microsoft.

Other browser vendors know this: Dart is never gonna fly outside of Google's walled garden.

It may not have to if Google can get (or has already) enough Chrome penetration.