This doesn't guarantee that the prototype assignment immediately follows. Also, what if classId denotes a non-function, such as a variable or an interface declaration? I'm thinking you should instead check that the prototype assignment is immediately contained in a statement list and that the immediately preceding statement is a function declaration.

You need to create this type once and cache it. Right now you're creating a brand new type every time which allocates a lot of objects and defeats all the type comparison caching we do.