Either understand const arrow assertions or give better errors on them

As mentioned by @threehams in https://github.com/microsoft/TypeScript/issues/35838#issuecomment-569832017, people are pretty confused about the rules for assertion functions, and I don't blame them. For example

const assert = (blah: unknown): asserts blah => { throw "hai"; }

let x: string | undefined;;
assert(x);

In our nightly releases, we give an elaboration on the declaration of assert like

'assert' needs an explicit type annotation.

For all intents and purposes, it looks like assert does have an annotation! The problem is that assert itself needs a : (x: unknown) => asserts x, because it's not automatically inferred from the arrow function.

We have two options:

• Remove this restriction when the declaration of an assertion function is trivially detectable.

• Make assertion function errors more clear for arrow functions:

  'assert' needs an explicit type annotation. While the function it has been assigned to is fully annotated, the variable declaration for 'assert' does not.
  

No se indica ningún archivo del repositorio ni ninguna prueba. Reproduce el ejemplo de aserción de flecha const del issue en una compilación nightly de TypeScript y, después, inspecciona la ruta de diagnóstico existente para funciones de aserción. Se considera terminado si las aserciones de flecha triviales de detectar se aceptan o si el diagnóstico explica que la propia declaración de variable carece de la anotación de tipo explícita requerida.