Couldn't it also be the case that he's not doing it because he's paid to, but because he loves Dart? People at Google can move around between projects, and generally, if you're working on a particular project, it's not because you are disinterested in it and simply collecting a paycheck, but you're enthusiastic about it.
This is especially true of developer advocates. No one's whining about all the HTML5 advocacy that Google does on sites like html5rocks.com, do you think the developers that write those articles merely do so because orders came down from above, or because they chose things they thought were exciting or that they were enthusiastic about.
I do not work at Google and yes, I love Dart. By following Seth's work for two years and a half, it is obvious to me that he loves Dart.
Why do I love Dart? As a developer and a university professor, for the first time in my long career, I can do the following with Dart:
I can use Dart both on the client and on the server;
with Dart, I can apply both object-oriented and functional way of programming.
I can develop in Dart and deploy applications in JavaScript.
I can be a productive developer with many Dart tools and libraries, and get a very good performance in either Dart applications or their JavaScript versions.
I can start developing a prototype without data types and introduce them when I need to convert the prototype to a deployable application.
I can use Dart for both synchronous and asynchronous programming.
I can use many publicly available packages and reuse their libraries.
I can be a web engineer on the client-side and a software engineer on the server-side, with the same language and many reusable libraries.
This is especially true of developer advocates. No one's whining about all the HTML5 advocacy that Google does on sites like html5rocks.com, do you think the developers that write those articles merely do so because orders came down from above, or because they chose things they thought were exciting or that they were enthusiastic about.