Unsupported private class error doesn't throw on building library for production

# 🐞 Bug report

### Command (mark with an `x`)

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

### Is this a regression?

i'm not sure

### Description

when i export a component in the library module but not in the public API
the build command throw errors only in normal mode, but in production mode it completes successfully

## 🔬 Minimal Reproduction

clone https://github.com/robertIsaac/test-library
and run ng b and ng b --prod

or generate from scratch

```
ng new test-library --create-application=false
cd test-library
ng generate library test-library
ng g c test-export --export
ng b
ng b --prod
```

## 🔥 Exception or Error
both commands should throw the following error

Unsupported private class TestExportComponent. This class is visible to consumers via TestLibraryModule -> TestExportComponent, but is not exported from the top-level library entrypoint.

## 🌍 Your Environment

$ ng version


     _                      _                 ____ _     ___

    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|

   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |

  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |

 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|

                |___/

    

Angular CLI: 9.1.1

Node: 12.14.1

OS: win32 x64

Angular: 9.1.2

... animations, common, compiler, compiler-cli, core, forms

... language-service, platform-browser, platform-browser-dynamic

... router

Ivy Workspace: Yes

Package                            Version

------------------------------------------------------------

@angular-devkit/architect          0.901.1

@angular-devkit/build-angular      0.901.1

@angular-devkit/build-ng-packagr   0.901.1

@angular-devkit/build-optimizer    0.901.1

@angular-devkit/build-webpack      0.901.1

@angular-devkit/core               9.1.1

@angular-devkit/schematics         9.1.1

@angular/cli                       9.1.1

@ngtools/webpack                   9.1.1

@schematics/angular                9.1.1

@schematics/update                 0.901.1

ng-packagr                         9.1.1

rxjs                               6.5.5

typescript                         3.8.3

webpack                            4.42.0

**Anything else relevant?**

it would be nice if --export command also update the public api file if it's running inside a library

Inizia riproducendo la build della libreria con `ng b` e `ng b --prod` usando i comandi e il repository della libreria di test descritti nell’issue. Confronta gli entry point delle build normale e di produzione per quanto riguarda l’API pubblica della libreria e la validazione delle classi private; il lavoro è completato quando entrambi i comandi riportano l’errore Unsupported private class.

Scritto dal modello di indicizzazione a partire dal testo della issue.