mark enableProdMode as deprecated and output a warning that it should not be used anymore

[sod]

angular 9.1.9

enableProdMode() right now is a placebo. The only thing it does is removing the app is running in dev mode warning to the console. So if you forgot to disable it at build time but rely on enableProdMode() to be working, your app runs in dev mode with a rather big performance hit.

You could print a warning, that enableProdMode() is deprecated and won't work and that dev mode can only be disabled at build time (or do (window as any).ngDevMode = false; as workaround). Or restore its functionality, if this is not intended.

Reproduction: https://angular-ivy-bjsurx.stackblitz.io/ | see main.ts, enableProdMode() is called
Open devtools and set a breakpoint anywhere where ngDevMode is used, and observe it not being falsy. Or look at the DOM and see debug attributes & comments.

[sod]

Seems like it's not totally noop. It's used frequently throughout angular. Only the new ivy dev mode https://github.com/angular/angular/blob/d1ea1f4c7f3358b730b0d94e65b00bc28cae279c/packages/core/src/util/ng_dev_mode.ts is being ignored.

Just wondered as the documentation says, that it turns off assertions and other checks, but those assertions are behind the ngDevMode variable as seen in the first screenshot.

[sod]

closed by #40124

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