I'm a bit paranoid about upgrading after breaking changes in 3.9 removed the ability to stub ES imports with Sinon/Jasmine/etc..., since the section of the release notes made it sound pretty innocuous: "Getters/Setters are No Longer Enumerable".
I have no idea, I don't have experience with Babel. As mentioned in the release notes, this change makes TS conform more closely to ECMAScript, so I don't think it's a bad decision. But it would've been nice to get some warning or error when trying to stub an ES import, rather than the silent failures