Allow comparing anything with null or undefined even when strict null checks are enabled

[malibuzios]

TypeScript Version:

1.9.0-dev.20160503

Code:

export function safeLibraryFunction1(arg: string) {
    if (arg == null)
        throw new TypeError(`Hi JS Consumer, I checked, it turns out the argument you 
                             gave was a null or undefined`);

    if (typeof arg !== "string")
        throw new TypeError(`Hi JS Consumer, I checked, the argument you gave me wasn't a string.. 
                             seems like something went wrong in your program?..`);
    // ..
}

interface SomeType {
    a: number;
}

export function safeLibraryFunction2(arg: SomeType) {
    if (arg === undefined)
        throw new TypeError(`Hi TS Consumer, it turns out the argument was undefined.
                             I don't assume you're using strict nullability checks,
                             I know that could be a bit difficult thing to introduce
                             or you haven't even heard about it 
                             so I also check at runtime :)`);
    if (arg === null)
        throw new TypeError(`Hi TS Consumer, it turns out the argument was null, 
                             maybe you had an unsafe cast somewhere in your program
                             or you're not using strict nullability checks?
                             I already thought of that so I checked anyway ;)`);
    // ..
}

export function safeLibraryFunction3(arg: SomeType) {
    if (arg != null) {
        //.. do something
    }
    else {
        throw new TypeError(`Hi JS Consumer, I think about you all the time
                             that's why I put many safeguards in my program
                             to make sure that everything goes smoothly..
                             c'ya later..`);
    }

}

Expected behavior:
The comparisons work.

Actual behavior:

Operator `==` cannot be applied to type X and null

Operator `===` cannot be applied to type X and null

Operator `===` cannot be applied to type X and undefined

Operator `!=` cannot be applied to type X and null

(My projects are full of these errors when strictNullChecks is enabled)

[ahejlsberg]

Yup, this one is already on our list.