I am really sad that TypeScript gave up on their spec. It previously was one of the strengths of the language that I was happy to cite. I guess it's a common crutch for languages (Python and so on) to say "the implementation is the spec" but without a spec to guide you, you lose sight of which features are intentional or accidents or even what the distinction between the two are.
The Closure library does has some great stuff in it. It's the accumulation of years of experience of working around lots of really subtle issues in lot of browsers. On the other side it also has a lot of code for issues that are obsolete, and it's hard to know which pieces are for what.
We used it at Lucid when we converted our codebase from Closure annotated JavaScript to TypeScript. https://www.lucidchart.com/techblog/2017/11/16/converting-60...
Google has support for using TypeScript in Closure annotated JavaScript via Clutz: https://github.com/angular/tsickle
It also has support converting TypeScript to Closure annotated JavaScript via tsickle: https://github.com/angular/clutz
Before we converted all of our codebase to TypeScript we used both Clutz and tsickle. It was complicated to setup, but it worked. Now we just use tsickle. It's great to get the optimization of the Closure Compiler while using TypeScript.
I agree, it would be cool for someone to do the same thing for Typescript, I seem to remember a repo a while back on Github that said someone was experimenting with it - cant for the life of me remember where though!
It'll probably happen anyway, but the current competition will determine which tool that will be. So far TS looks like it's winning it, but, clearly, not by a large enough margin that the industry coalesces around it - yet.
Another option would be to add TypeScript support to Closure Compiler, although it doesn't seem likely since there's no spec to guarantee compatibility.
I'd love to see Facebook, Google, and Microsoft team up in this space, instead of creating 3 separate but very highly similar tools.
Also, while we're talking about Closure, let's take a moment to appreciate its amazing UI toolkit [0]. I'd still consider many of their widgets to be the gold standard. It's written with desktop clients in mind and is incredibly feature-complete. I believe it also has pretty good accessibility, although I haven't personally tried that out. Oh, and don't forget the i18n as well! The closure library definitely has a lot of quirky aspects to it, but it's still quite amazing if you consider its age and how much stuff it supports. It's worth taking a few minutes to browse through their docs [1].
[0] https://google.github.io/closure-library/source/closure/goog...
[1] https://google.github.io/closure-library/api/