Clean up sourcemap

[guybedford]

This cleans up the source map output such that instead of getting:

/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/webpack:/test.js:4
throw new Error('test');
^
Error: test
    at Object.294 (/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/webpack:/test.js:4:1)
    at __webpack_require__ (/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/webpack:/webpack/bootstrap:19:1)
    at startup (/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/webpack:/webpack/bootstrap:33:1)
    at /private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/webpack:/webpack/bootstrap:37:1
    at Object.<anonymous> (/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/index.js:43:10)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)

we will now get:

/test.js:4
throw new Error('test');
^
Error: test
    at Object.294 (/test.js:4:1)
    at __webpack_require__ (/webpack:bootstrap:19:1)
    at startup (/webpack:bootstrap:33:1)
    at /webpack:bootstrap:37:1
    at Object.<anonymous> (/private/var/folders/bm/grflgmxs62g01cd9m8sjjm740000gn/T/d41d8cd98f00b204e9800998ecf8427e/index.js:43:10)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)

[codecov-io]

Codecov Report

Merging #269 into master will increase coverage by 0.26%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
+ Coverage   74.23%   74.49%   +0.26%     
==========================================
  Files          17       17              
  Lines         885      898      +13     
==========================================
+ Hits          657      669      +12     
- Misses        228      229       +1

Impacted Files	Coverage Δ
src/index.js	70.96% <92.3%> (+1.95%)	⬆️

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