Small TypeScript improvements

[Tim Van der Lippe]

Hey devtools-dev,

Today two small TypeScript improvements have landed:

1. Return types of functions/method declarations in the `front_end/` are now required by ESLint. This change reflects the internal TypeScript style guide. We are working on enabling this rule for non-`front_end/` code as well.

2. We have added a new function called `assertNever` to Platform (https://crrev.com/c/2589953). This function can be used to enforce exhaustive switch statements during compile time. In the default-clause of a switch statement (given that all values of an enum/union have been handled), TypeScript infers the switched-value to be of type `never`. As such, you can call `assertNever` with its value to ensure the type remains `never`. When you add new values to the enum/union and you forget to handle it in the switch branches, the compiler will throw an error. For additional examples, see https://crrev.com/c/2589954

Let us know if you run into any issues.

Cheers,

LON

[Connor Clark]

I'm curious if an eslint rule would be better than code modifications required by 2. A quick google search shows this: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/switch-exhaustiveness-check.md 

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[Tim Van der Lippe]

Ideally we would make use of such a rule. However, there are (a majority of) instances where our switch-statements are intentionally non-exhaustive (about 5 out of ~25 were intentionally exhaustive). By using `assertNever`, we provide the author the opportunity to decide what to do, while also making sure that on Runtime we have a descriptive error (if anything went wrong with version mismatches).