The `ngDevMode` and `ngI18nClosureMode` are special in that they should be set to `false` on production builds in order to shake out code associated with it.
Angular CLI does this in 5fc1f2499c/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts (L279-L282)
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But in https://github.com/angular/angular/pull/28689 the toplevel usage was changed from `ngDevMode` to `global['ngDevMode']` (and the same for `ngI18nClosureMode`). This indirection prevents the static analysis in Terser from effecting the replacement.
PR Close #29929