This lets TypeScript narrow types when it can, making type checking and
code completion a bit more useful

For example:

```
function makeNumber(x: number | Long): number {
  if (isLong(x)) {
    // If isLong wasn't a type guard, this would cause a compilation
    // error, since TS wouldn't know that x was specifically a Long.
    return x.toNumber()
  }
  return x
}
```

For more info:

  https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards