you mean using TypeScript as the glue for Dart when developing in TS/JS ? like WinMDExp did for C# and C++/CX ?
Actually I'm thinking about another use of TypeScript, as a metadata glue of JS, for consuming js in C#. like how WinMD file works for C# to consume WinRT.
One usage of this is to make the WinRT circle complete, currently JS is consuming only, while C# and C++ can create components for others to use.
There is a tool for winmd => ts, maybe they can also do it the other way.
Another usage of this could be, if C# needs to interact with JS code, either on the client or server side, either as a CLR language or go through IL2JS, if its needed somehow, it can integrate with existing JS libraries, using TS as the metadata.