fix(core): export inject() from @angular/core (#22389)

inject() supports the ngInjectableDef-based configuration of the injector
(otherwise known as tree-shakeable services). It was missing from the
exported API of @angular/core, this PR adds it.

The test added here is correct in theory, but may pass accidentally due
to the decorator side-effect replacing the inject() call at runtime. An
upcoming compiler PR will strip reified decorators from the output
entirely.

Fixes #22388

PR Close #22389

packages/examples/core/di/ts/injector_spec.ts

@@ -6,7 +6,25 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {InjectionToken, Injector, ReflectiveInjector} from '@angular/core';
+import {APP_ROOT_SCOPE, InjectFlags, InjectionToken, Injector, ReflectiveInjector, Type, inject, ɵsetCurrentInjector as setCurrentInjector} from '@angular/core';
+
+class MockRootScopeInjector implements Injector {
+  constructor(readonly parent: Injector) {}
+
+  get<T>(
+      token: Type<T>|InjectionToken<T>, defaultValue?: any,
+      flags: InjectFlags = InjectFlags.Default): T {
+    if ((token as any).ngInjectableDef && (token as any).ngInjectableDef.scope === APP_ROOT_SCOPE) {
+      const old = setCurrentInjector(this);
+      try {
+        return (token as any).ngInjectableDef.factory();
+      } finally {
+        setCurrentInjector(old);
+      }
+    }
+    return this.parent.get(token, defaultValue, flags);
+  }
+}
 
 {
   describe('injector metadata examples', () => {
@@ -37,5 +55,25 @@ import {InjectionToken, Injector, ReflectiveInjector} from '@angular/core';
       expect(url).toBe('http://localhost');
       // #enddocregion
     });
+
+    it('injects a tree-shaekable InjectionToken', () => {
+      class MyDep {}
+      const injector = new MockRootScopeInjector(ReflectiveInjector.resolveAndCreate([MyDep]));
+
+      // #docregion ShakeableInjectionToken
+      class MyService {
+        constructor(readonly myDep: MyDep) {}
+      }
+
+      const MY_SERVICE_TOKEN = new InjectionToken<MyService>('Manually constructed MyService', {
+        scope: APP_ROOT_SCOPE,
+        factory: () => new MyService(inject(MyDep)),
+      });
+
+      const instance = injector.get(MY_SERVICE_TOKEN);
+      expect(instance instanceof MyService).toBeTruthy();
+      expect(instance.myDep instanceof MyDep).toBeTruthy();
+      // #enddocregion
+    });
   });
 }