To get promoted at Google, you need to demonstrate project complexity so engineers is required to build new frameworks. Is it quite risky to adopt an AdWords framework (like GWT) which has high chance of being deprecated?
As I mentioned in my longer post, I have considered alternatives. Dart's position was strengthened because the code analyzer is very advanced and easy to use. On the low-probability scenario where Dart development would cease, and I'm left with an important Dart codebase, I would have good chances that I could automatically transpile most of my app to a new stack (assuming I'm using enough type annotations).
However, as I follow Dart development and community closely, I believe there is very little risk of getting it abandoned anytime soon.