Empty source map with localization and --watch

[richtems]

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• xi18n
• run
• config
• help
• version
• doc

Is this a regression?

Yes, the previous version in which this bug was not present was: 8.2.2

Description

When using @angular/localization and building with --watch, the main.js source map is empty.

🔬 Minimal Reproduction

ng new TestApp
cd .\TestApp\
ng add @angular/localize

Add i18n to project in angular.json:

"projects": {
  ...
  "TestApp": {
    ...
    "i18n": {
      "sourceLocale": "en-US",
      "locales": {
        "fr": "src/locale/messages.fr.xlf"
      }
    }
    ...
    "architect": {
      "build": {
          ...
          "options": {
            ...
            "localize": ["fr"],

ng xi18n
copy messages.xlf to "src/locale/messages.fr.xlf"
ng build --watch

Now the content of dist/TestApp/fr/main.js.map is:
{}

🌍 Your Environment

Angular CLI: 9.0.4
Node: 10.15.1
OS: win32 x64

Angular: 9.0.4
... animations, cli, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router
Ivy Workspace: Yes

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.900.4
@angular-devkit/build-angular     0.900.4
@angular-devkit/build-optimizer   0.900.4
@angular-devkit/build-webpack     0.900.4
@angular-devkit/core              9.0.4
@angular-devkit/schematics        9.0.4
@ngtools/webpack                  9.0.4
@schematics/angular               9.0.4
@schematics/update                0.900.4
rxjs                              6.5.4
typescript                        3.7.5
webpack                           4.41.2

Anything else relevant?

Might be related to #16526 ?

[HitkoDev]

This also happens without --watch, as long as localize option is enabled. However, chunks without any localized content still have the correct source map. I believe angular/angular#33123 was supposed to solve this, but it's been closed and there's not yet an alternative proposed.

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