feat(ivy): use i18n locale data to determine the plural form of ICU expressions (#29249)

Plural ICU expressions depend on the locale (different languages have different plural forms). Until now the locale was hard coded as `en-US`.
For compatibility reasons, if you use ivy with AOT and bootstrap your app with `bootstrapModule` then the `LOCALE_ID` token will be set automatically for ivy, which is then used to get the correct plural form.
If you use JIT, you need to define the `LOCALE_ID` provider on the module that you bootstrap.
For `TestBed` you can use either `configureTestingModule` or `overrideProvider` to define that provider.
If you don't use the compat mode and start your app with `renderComponent` you need to call `ɵsetLocaleId` manually to define the `LOCALE_ID` before bootstrap. We expect this to change once we start adding the new i18n APIs, so don't rely on this function (there's a reason why it's a private export).
PR Close #29249

packages/core/test/acceptance/BUILD.bazel

@@ -13,6 +13,7 @@ ts_library(
         "//packages/animations/browser",
         "//packages/animations/browser/testing",
         "//packages/common",
+        "//packages/common/locales",
         "//packages/compiler",
         "//packages/compiler/testing",
         "//packages/core",

packages/core/test/acceptance/i18n_spec.ts

@@ -6,7 +6,9 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Component, ContentChild, ContentChildren, Directive, HostBinding, QueryList, TemplateRef, Type, ViewChild, ViewContainerRef, ɵi18nConfigureLocalize} from '@angular/core';
+import {registerLocaleData} from '@angular/common';
+import localeRo from '@angular/common/locales/ro';
+import {Component, ContentChild, ContentChildren, Directive, HostBinding, LOCALE_ID, QueryList, TemplateRef, Type, ViewChild, ViewContainerRef, ɵi18nConfigureLocalize} from '@angular/core';
 import {TestBed} from '@angular/core/testing';
 import {expect} from '@angular/platform-browser/testing/src/matchers';
 import {onlyInIvy} from '@angular/private/testing';
@@ -549,6 +551,46 @@ onlyInIvy('Ivy i18n logic').describe('runtime i18n', () => {
       expect(fixture.nativeElement.innerHTML)
           .toEqual(`<div>4 animaux<!--nested ICU 0-->!<!--ICU 5--></div>`);
     });
+
+    it('should return the correct plural form for ICU expressions when using a specific locale',
+       () => {
+         registerLocaleData(localeRo);
+         TestBed.configureTestingModule({providers: [{provide: LOCALE_ID, useValue: 'ro'}]});
+         // We could also use `TestBed.overrideProvider(LOCALE_ID, {useValue: 'ro'});`
+         const fixture = initWithTemplate(AppComp, `
+          {count, plural,
+            =0 {no email}
+            =one {one email}
+            =few {a few emails}
+            =other {lots of emails}
+          }`);
+
+         expect(fixture.nativeElement.innerHTML).toEqual('no email<!--ICU 2-->');
+
+         // Change detection cycle, no model changes
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('no email<!--ICU 2-->');
+
+         fixture.componentInstance.count = 3;
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('a few emails<!--ICU 2-->');
+
+         fixture.componentInstance.count = 1;
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('one email<!--ICU 2-->');
+
+         fixture.componentInstance.count = 10;
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('a few emails<!--ICU 2-->');
+
+         fixture.componentInstance.count = 20;
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('lots of emails<!--ICU 2-->');
+
+         fixture.componentInstance.count = 0;
+         fixture.detectChanges();
+         expect(fixture.nativeElement.innerHTML).toEqual('no email<!--ICU 2-->');
+       });
   });
 
   describe('should support attributes', () => {