Source map error with RC version update

[JSmithOner]

🐞 Bug report

Command (mark with an x)

• serve
• update

Is this a regression?

Yes

Description

A clear and concise description of the problem...

when connecting to http://localhost:4200 after ng serve I get a blank screen page and an error

🔬 Minimal Reproduction

Maybe not the best idea I had but I've runned the following command
all of the following packages where on 8 version

ng update @angular/cdk @angular/cli @angular/core @angular/material rxjs --allowDirty --next --force

now my angular.json looks like that:

  "dependencies": {
    "@angular/animations": "^11.0.0-rc.0",
    "@angular/cdk": "^11.0.0-rc.0",
    "@angular/common": "^11.0.0-rc.0",
    "@angular/compiler": "^11.0.0-rc.0",
    "@angular/core": "^11.0.0-rc.0",
    "@angular/fire": "^6.0.2",
    "@angular/forms": "^11.0.0-rc.0",
    "@angular/material": "^11.0.0-rc.0",
    "@angular/platform-browser": "^11.0.0-rc.0",
    "@angular/platform-browser-dynamic": "^11.0.0-rc.0",
    "@angular/router": "^11.0.0-rc.0",
    "@types/fabric": "^3.0.0",
    "bootstrap": "^4.3.1",
    "color": "^3.1.2",
    "core-js": "^2.5.4",
    "fabric": "^3.3.2",
    "fabric-with-gestures": "^4.0.0-rc.1",
    "firebase": "^7.19.1",
    "jquery": "^1.9.1",
    "popper.js": "^1.14.7",
    "rxjs": "^7.0.0-beta.8",
    "tslib": "^1.9.0",
    "zone.js": "^0.10.3"
  },
  "devDependencies": {
    "@angular-devkit/build-angular": "^0.1100.0-rc.0",
    "@angular/cli": "~11.0.0-rc.0",
    "@angular/compiler-cli": "~11.0.0-rc.0",
    "@angular/language-service": "~11.0.0-rc.0",
    "@types/jasmine": "~2.8.8",
    "@types/jasminewd2": "~2.0.3",
    "@types/jquery": "^3.3.31",
    "@types/node": "^8.9.5",
    "codelyzer": "~4.5.0",
    "jasmine-core": "~2.99.1",
    "jasmine-spec-reporter": "~4.2.1",
    "karma": "~5.2.3",
    "karma-chrome-launcher": "~2.2.0",
    "karma-coverage-istanbul-reporter": "~2.0.1",
    "karma-jasmine": "~1.1.2",
    "karma-jasmine-html-reporter": "^0.2.2",
    "protractor": "~7.0.0",
    "ts-node": "~7.0.0",
    "tslib": "^1.9.0",
    "tslint": "~6.1.3",
    "typescript": "~4.0.5"
  }

🔥 Exception or Error

  
    Error: request failed with status 404
    ressource URL: http://localhost:4200/vendor.js
    URL source link: startWith.js.map
  

🌍 Your Environment

Angular CLI: 11.0.0-rc.0
Node: 14.5.0
OS: darwin x64

Angular: 11.0.0-rc.0
... animations, cdk, cli, common, compiler, compiler-cli, core
... forms, language-service, material, platform-browser
... platform-browser-dynamic, router
Ivy Workspace: Yes

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1100.0-rc.0
@angular-devkit/build-angular   0.1100.0-rc.0
@angular-devkit/core            11.0.0-rc.0
@angular-devkit/schematics      11.0.0-rc.0
@angular/fire                   6.0.2
@schematics/angular             11.0.0-rc.0
@schematics/update              0.1100.0-rc.0
rxjs                            7.0.0-beta.8
typescript                      4.0.5

Anything else relevant?

The error message used to show on Chrome but the one I gave you is a translation of Firefox and don't show on Chrome anymore.
Firefox 82.0.1
Chrome 86.0.4240.111

[alan-agius4]

In a nutshell what is happening here is that.

In version 11, we no longer process vendor sourcemaps unless the vendor option is enabled which improves build time, ~3 seconds on an ng new application. The drawback of this that //# sourceMappingURL comment is not removed from the source file of the libraries. Hence, a warning will be issued in the console because the sourcemap cannot be loaded.