No easy way to convince TypeScript a variable has been initialized.

Reported by @wycats. Compile this under `strictNullChecks` and `noImplicitAny`.

```ts
declare function someAbstraction(x: any): any;

let x: string;

someAbstraction(() => {
  x = "hello";
});

// All of the following are errors.
x;
x as any;
x!;
```

I'd argue that both type assertions should be able to signal to the type system that the variable has been initialized.