I've tested your example and sure: That seems like a bug in the declaration of the Promise-class.
What I like about TypeScript and which I think is "sound" compared to Flow is the whole experience and how it is backed by good tooling.
With TypeScript I can open almost "any" editor and expect it to understand and respect the typings of all my TypeScript code. It will correctly autocomplete, and tell me when I'm using undefined members.
Basically with TypeScript every part of the stack seems to work for me, the developer, to ensure I make the right decisions and get the assistance I need, at all time. That makes it feel like a very safe and sound language to write code in.
With flow? Well fuck that. From what I can tell, you get almost nothing.
You can run your compiler and see if it validates, but I've seen almost no editor-support worth mentioning, meaning that while the type-inference in the compiler may occasionally be better (but for how long? TS has a ton of momentum), the overall experience feels much less sound and much more fragile. Much more unsafe.
The type-declarations definitely feel as important as the way they are declared: they're mere comments. They're not real.
