fix(core): use shakeable global definitions (angular#29929)

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 https://github.com/angular/angular-cli/blob/5fc1f2499cbe57f9a95e4b0dfced130eb3a8046d/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts#L279-L282.

But in angular#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 angular#29929

Author: filipesilva

packages/core/src/util/ng_dev_mode.ts

@@ -82,7 +82,10 @@ export function ngDevModeResetPerfCounters(): NgDevModePerfCounters {
  * The idea is that unless we are doing production build where we explicitly
  * set `ngDevMode == false` we should be helping the developer by providing
  * as much early warning and errors as possible.
+ *
+ * NOTE: changes to the `ngDevMode` name must be synced with the CLI and
+ * possibly other third party tooling. Check with them before altering it.
  */
-if (typeof global['ngDevMode'] === 'undefined' || global['ngDevMode']) {
+if (typeof ngDevMode === 'undefined' || ngDevMode) {
   ngDevModeResetPerfCounters();
 }

packages/core/src/util/ng_i18n_closure_mode.ts

@@ -12,7 +12,11 @@ declare global {
   const ngI18nClosureMode: boolean;
 }
 
-if (typeof global['ngI18nClosureMode'] === 'undefined') {
+/**
+ * NOTE: changes to the `ngI18nClosureMode` name must be synced with the CLI and
+ * possibly other third party tooling. Check with them before altering it.
+ */
+if (typeof ngI18nClosureMode === 'undefined') {
   // Make sure to refer to ngI18nClosureMode as ['ngI18nClosureMode'] for closure.
   global['ngI18nClosureMode'] =
       // TODO(FW-1250): validate that this actually, you know, works.