Source map doesn't work

[TingOOOgniT]

Is this a bug report?

Yes

Did you try recovering your dependencies?

I use create-react-app to create a project and run yarn start without any change. But I cannot find source files under localhost:3000

Which terms did you search for in User Guide?

I have tried to search this problem but I just cannot find something relevant.

Environment

Environment Info:

System:
OS: macOS 10.14.3
CPU: x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
Binaries:
Node: 8.11.3 - /usr/local/bin/node
Yarn: 1.13.0 - /usr/local/bin/yarn
npm: 5.6.0 - /usr/local/bin/npm
Browsers:
Chrome: 72.0.3626.96
Firefox: 60.4.0
Safari: 12.0.3
npmPackages:
react: ^16.8.1 => 16.8.1
react-dom: ^16.8.1 => 16.8.1
react-scripts: 2.1.5 => 2.1.5
npmGlobalPackages:
create-react-app: 1.5.2

Steps to Reproduce

(Write your steps here:)

1. yarn create react-app react-app
2. cd react-app
3. yarn start

Expected Behavior

There should be a source code folder under localhost:3000 without being minified.

Actual Behavior

There are only code minified.

Reproducible Demo

(Paste the link to an example project and exact instructions to reproduce the issue.)

[TingOOOgniT]

Only can find source code under webpack-internal://
I used to create a react app with react-script 1.1.4 and I can find a folder under localhost:3000 with all source code in it.

[Jipperism]

Same issue here.

[bugzpodder]

This should be fixed by: #6444