The story as I heard/read somewhere I cannot at this moment recall (InfoQ, conference networking, Podcast, blog,...), goes like this.

Many of the important devs on the V8 team, like Gilad Bracha, were getting tired of working on the project and started to look elsewhere for a career change.

Allowing them to create their own vision what a JavaScript replacement might look like (aka Dart) motivated them to stay on the team and at Google.

When the Chrome team decided that they wouldn't be merging the Dart VM into Chrome at all and would rather focus on JavaScript, including their efforts for adding type annotations to JavaScript, the Dart team was going to be assigned new tasks.

However in the meantime the AdSense team had started using Dart and they enjoyed using it quite a lot, so a decision was made to move the Dart team away from Chrome into supporting the AdSense team as main role.

If this is not true, I would welcome any Googler that knows better, to correct me.