Fix sourcemap chaining

[guybedford]

This fixes the loaders to support sourcemap chaining properly, and includes a simple integration test to show the TypeScript sourcemaps are that the right offset.

Fixes #353

There will likely be more work to be done here in due course, which we can improve over time (in terms of ensuring the fidelity of the source maps, not their accuracy).

Prerequisite on vercel/webpack-asset-relocator-loader#21.

[guybedford]

We should also keep an eye on performance with this change.

[codecov-io]

Codecov Report

Merging #355 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #355   +/-   ##
=======================================
  Coverage   74.61%   74.61%           
=======================================
  Files          12       12           
  Lines         386      386           
=======================================
  Hits          288      288           
  Misses         98       98

Impacted Files	Coverage Δ
src/index.js	84.47% <ø> (ø)	⬆️
src/loaders/uncacheable.js	100% <100%> (ø)	⬆️
src/loaders/empty-loader.js	100% <100%> (ø)	⬆️

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[styfle]

So the column number is still wrong (1), even after changing devtool?

[styfle]

@guybedford I just checked out this branch and built the code to see how it would work with #353 and it is actually worse.

git clone https://github.com/styfle/ncc-bug-stack
cd ncc-bug-stack
git checkout ts
yarn
rm -rf dist && ../ncc/dist/ncc/cli.js build index.ts --source-map
node dist

Visit http://localhost:3000

Expected: ncc-bug-stack/index.ts:4

Actual: ncc-bug-stack/dist/index.js:61

[guybedford]

@styfle interesting, must have changed since my first tests. I've pushed up a patch with a fix.

[styfle]

First I ran this in the ncc directory

git pull && rm -rf node_modules && yarn && yarn build

Then I ran this in the ncc-bug-stack directory

rm -rf dist && ~/Code/zeit/ncc/dist/ncc/cli.js cache clean && ~/Code/zeit/ncc/dist/ncc/cli.js build index.ts --source-map && node dist

I still see the same (incorrect) error:

Error: Should throw
    at Server.handler (/Users/styfle/Code/foo/ncc-bug-stack/dist/index.js:61:19)
    at Server.emit (events.js:189:13)
    at parserOnIncoming (_http_server.js:676:12)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:109:17)

I even tried disabling chrome caching 🤷‍♂️

[styfle]

Ok so change tsconfig.json to enable sourceMap:true fixed the problem.

We'll have to make sure that user's enable this feature in their TS projects.