feat(testing/asserts): Use assertion signature for "assertExists"

[jsejcksn]

This is an improvement which allows using the function as a type guard in addition to its runtime safety. See the following TS playground link for a comparison of before/after: TS Playground

The contributing guidelines state to make sure that changes are covered by a test. However, I don't see any testing framework here for types. If I modify an existing test to include a case which requires calling the function to narrow a type before use, then any type-incompatible changes to the function later would later cause a compilation error of the test, rather than a runtime error during testing... but I'm not sure that kind of meta behavior is what the guidelines mean.

[CLAassistant]

All committers have signed the CLA.

[kt3k]

Thank you for your suggestion! I'm in favor of this change.

As regards to testing, how about adding the lines from your example in TS playground as a test case?

const value = new URLSearchParams({value: 'test'}).get('value');
assertExists(value);
console.log(value.length);

I think this test case proves that the type guard & type predicate work.

[kt3k]

The change looks almost good to me, but I'm not 100% sure this doesn't break any user code. Is it possible that typescript throws errors like Can not infer T in assertExists<T>? cc @kitsonk

[kt3k]

LGTM. Thanks! @jsejcksn