Apparently this is just type checking. It's not going to do any dead code removal like Closure Compiler in advanced mode or provide a better syntax like TypeScript. Whether that's good or bad depends on what you're looking for.
The React.js team has been explicit about the library being GCC advanced mode compatible, so they certainly have awareness of its capabilities. Whether they are using them together internally or they use a different solution for tree shaking et al is another question.