Allow packages to opt-in to typings-checker instead of tsc

[danvk]

Just throwing this out there to get some feedback. See DefinitelyTyped/DefinitelyTyped#1572 (comment)

For packages that opt-in, this would let them run their tests through typings-checker instead of tsc. typings-checker lets allows errors to be marked as expected and lets tests make assertions about the types of symbols. This helps prevent unintended any or {} types from creeping in.

[danvk]

See also danvk/typings-checker#18 — it might be easier to set up typings-checker as something that runs in addition to tsc rather than instead of it (just like tslint).

[ghost]

I'm working on a new tester at https://github.com/andy-ms/dtslint (eventually this will be moved to https://github.com/Microsoft/dtslint). This will allow us to have just one tool instead of types-publisher + tsc + tslint + typings-checker. It includes a conversion of typings-checker into a lint rule at https://github.com/andy-ms/dtslint/blob/master/src/rules/expectRule.ts (Update: https://github.com/Microsoft/dtslint/blob/master/src/rules/expectRule.ts). Would this be OK?

I also removed the ability for $ExpectError to specify an error string, because we want to be able to change error messages without breaking any tests. You had already put that functionality under a flag.

[danvk]

@andy-ms cool! Glad to hear the TS team is improving tooling around testing typings files.

Feel free to use whatever code you like, just be sure to mention me as a contributor.

[ghost]

Great! Closing in favor of #312.