Bump Typescript to 3.7.4

[skylerjokiel]

This is a new PR because we were blocked before by tslint, which was on a very old ts build) and it was easier to start over then merge in a month of changes. Thanks @tylerbutler for doing amazing work in December and moving us onto eslint 😄

3.7 has a bug around array types where it munges them if you use undefined, null or unknown. (Issue: 33752)[https://github.com/microsoft/TypeScript/issues/33752]. I reached out to Ryan Cavanaugh who's an EM on the TS team and he put it on the backlog to get it fixed for 3.8. Because we only only have once instance of it impacting us I think it's not too bad and I added a comment in that spot.

Original Comment

3.7 has added a few cool features that make life notably nicer.

Highlights

Optional Chaining

// Before
if (foo && foo.bar && foo.bar.baz) {
    // ...
}

// After-ish
if (foo?.bar?.baz) {
    // ...
}

Nullish Coalescing

let x = foo ?? bar();

Assertion Functions

function assertIsString(val: any): asserts val is string {
    if (typeof val !== "string") {
        throw new AssertionError("Not a string!");
    }
}

function yell(str: any) {
    assertIsString(str);

    // Now TypeScript knows that 'str' is a 'string'.

    return str.toUppercase();
    //         ~~~~~~~~~~~
    // error: Property 'toUppercase' does not exist on type 'string'.
    //        Did you mean 'toUpperCase'?
}

Build Changes

There are a bunch of changes to the tsc build with Project References that came in 3.0. This included --incremental.

With 3.7 they made this better by adding Build-Free Editing with Project Refrences. We haven't taken advantage of Project References yet but we might be able to offload some of our incremental build logic to tsc itself.

Entire list of thing can be found here:

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-5.html
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-6.html
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html