We discard a lot of useful location information when generating the compiler output — we can use that information to create better sourcemaps. In some cases this won't make much difference to the typical developer experience (it basically just means that the 'JS Output' tab of the playground will have more sensible mapping between left and right), but in other cases it's the difference between a useful stack trace and a useless one.

Bit more to do, including (ideally) some sourcemap tests, though they're finicky to create.

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