fix (#4199): let postcss pass-through sourcemap

[asnowwolf]

resolve issue #4199 CSS Sourcemap is empty.

[filipesilva]

@asnowwolf I think this was fixed in #4222. If that's incorrect, let me know.

[asnowwolf]

@filipesilva I think the root cause is post-css-loader lost the sourcemap which generated by sass-loader. I'm not sure that PR is resolved the issue. I'll test it later.

[asnowwolf]

@filipesilva
I just tested the latest master branch and it still lost the sourcemap (I turned on the sourcemap options)

[filipesilva]

Can you give me some repro steps to see that? I added a few tests in #4222 and they verify mappings are being generated.

[asnowwolf]

see https://github.com/asnowwolf/t1

the step is:

1. pull the latest master branch of angular-cli
2. npm link it as global package
3. ng new t1 --style=scss --extract-css
4. npm i --save bootstrap@4.0.0-alpha.6
5. add @import '~bootstrap/scss/bootstrap'; to style.scss
6. ng serve --souremap
7. in chrome, I can't see the sourcemap correctly.

[asnowwolf]

Sorry, I've tested it again, It's right now. I think there are some mistakes on my first test.

[filipesilva]

I was just testing myself and your demo seemed to give me the sourcemaps. I think you didn't npm link angular-cli to have the master version in the project.

Anyway, glad it's working for you!

[asnowwolf]

Sure, I think that's the mistake. thank you.

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