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

Start by reproducing the library build with ng b and ng b --prod using the commands and test-library repository described in the issue. Compare the normal and production build entry points around the library's public API and private-class validation; done means both commands report the Unsupported private class error.

🐞 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?

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