Vendor sourcemaps are not available in build-angular@0.12.1

[Tallyb]

Bug Report or Feature Request (mark with an x)

- [ X ] bug report -> please search issues before submitting
- [ ] feature request

Command (mark with an x)

- [ ] new
- [ ] build
- [ X ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

Versions

node v10.14.1
npm 6.5.0

Package Version

@angular-devkit/architect 0.12.1
@angular-devkit/build-angular 0.12.1
@angular-devkit/build-ng-packagr 0.12.1
@angular-devkit/build-optimizer 0.12.1
@angular-devkit/build-webpack 0.12.1
@angular-devkit/core 7.2.1
@angular-devkit/schematics 7.2.1
@angular/cli 7.2.1
@ngtools/json-schema 1.1.0
@ngtools/webpack 7.2.1
@schematics/angular 7.2.1
@schematics/update 0.12.1
ng-packagr 4.5.0
rxjs 6.3.3
typescript 3.2.2
webpack 4.23.1

Repro steps

Desired functionality: have sourcemaps...

[alan-agius4]

Duplicate of #13414

Will be solved in the next patch version

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