Invalid source map path for @angular/common/http/testing bundle

[Delagen]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

Cannot find SourceMap 'http__testing.js.map': Error: Can't
resolve './http__testing.js.map' in '...\node_modules@angul
ar\common\fesm5\http'

Expected behavior

No warning.

Minimal reproduction of the problem with instructions

Open file in npm bundle @angular/common/fesm5/http/testing.js
At the end of file //# sourceMappingURL=http__testing.js.map

What is the motivation / use case for changing the behavior?

Fix bug in build system

Environment

Angular version: 6.1.3

Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: 10.9.0
- Platform:  Windows

Others:

[IgorMinar]

I sent #27313 to fix this.. thanks for the report.

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