The source maps allow the browser to display original files in the
debugger and in the console.
By default, webpack only includes source maps generated in the current
build. It doesn't include source maps located in node_modules. The
source-map-loader npm packages allow webpack to discover third-party
source maps and the `"devtool": "eval-source-map"` strategy makes
webpack expose both loaded source maps and generated source maps to the
browser.