Add type tests for known type errors

[dmnd]

To make sure our typing is working, we could (a) brainstorm what kind of type errors we want to prevent then (b) ensure that all of these potential errors have an $ExpectError test that asserts we detect that type error.

The list (a) will also be useful for #22.

[zgotsch]

Also, set up infra for Typescript type tests using dtslint.