The migration to TS will probably occur on two different branches :

• what we can do on the 5.x branch should be done here, without altering the dist output,
• what can not be done here should be done on the 6.x branch.

A list of things already spotted that won't be possible on v5 :

• buble is our actual ES5 compiler, and does not works well with typescript output or even modern code (string interpolation, class props, etc.),
• In the immutable codebase, the class constructor are factories that can create instances of other classes. This is not possible in modern JS. (See Avoid return in constructors #2041 )

The purpose of that is to make the full migration the less risky possible by doing everything possible in the v5 branch.