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

まず、issue に記載されているコマンドとテストライブラリのリポジトリを使って、`ng b` と `ng b --prod` でライブラリのビルドを再現します。通常ビルドと Production ビルドのエントリーポイントについて、ライブラリの公開 API と private class の検証を比較します。両方のコマンドが Unsupported private class エラーを報告すれば完了です。

索引モデルが issue の本文から書いたものです。