feat: allow direct scoping of @Injectables to the root injector (#22185)

@Injectable() supports a scope parameter which specifies the target module.
However, it's still difficult to specify that a particular service belongs
in the root injector. A developer attempting to ensure that must either
also provide a module intended for placement in the root injector or target
a module known to already be in the root injector (e.g. BrowserModule).
Both of these strategies are cumbersome and brittle.

Instead, this commit adds a token APP_ROOT_SCOPE which provides a
straightforward way of targeting the root injector directly, without
requiring special knowledge of modules within it.

PR Close #22185

packages/core/src/di.ts

@@ -23,3 +23,4 @@ export {StaticProvider, ValueProvider, ExistingProvider, FactoryProvider, Provid
 export {ResolvedReflectiveFactory, ResolvedReflectiveProvider} from './di/reflective_provider';
 export {ReflectiveKey} from './di/reflective_key';
 export {InjectionToken} from './di/injection_token';
+export {APP_ROOT_SCOPE} from './di/scope';

packages/core/src/di/scope.ts

@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Type} from '../type';
+import {InjectionToken} from './injection_token';
+
+
+// APP_ROOT_SCOPE is cast as a Type to allow for its usage as the scope parameter of @Injectable().
+
+/**
+ * A scope which targets the root injector.
+ *
+ * When specified as the `scope` parameter to `@Injectable` or `InjectionToken`, this special
+ * scope indicates the provider for the service or token being configured belongs in the root
+ * injector. This is loosely equivalent to the convention of having a `forRoot()` static
+ * function within a module that configures the provider, and expecting users to only import that
+ * module via its `forRoot()` function in the root injector.
+ *
+ * @experimental
+ */
+export const APP_ROOT_SCOPE: Type<any> = new InjectionToken<boolean>(
+    'The presence of this token marks an injector as being the root injector.') as any;

packages/core/src/view/ng_module.ts

@@ -8,6 +8,7 @@
 
 import {resolveForwardRef} from '../di/forward_ref';
 import {InjectFlags, Injector, setCurrentInjector} from '../di/injector';
+import {APP_ROOT_SCOPE} from '../di/scope';
 import {NgModuleRef} from '../linker/ng_module_factory';
 import {stringify} from '../util';
 
@@ -43,8 +44,12 @@ export function moduleProvideDef(
 export function moduleDef(providers: NgModuleProviderDef[]): NgModuleDefinition {
   const providersByKey: {[key: string]: NgModuleProviderDef} = {};
   const modules = [];
+  let isRoot: boolean = false;
   for (let i = 0; i < providers.length; i++) {
     const provider = providers[i];
+    if (provider.token === APP_ROOT_SCOPE) {
+      isRoot = true;
+    }
     if (provider.flags & NodeFlags.TypeNgModule) {
       modules.push(provider.token);
     }
@@ -57,6 +62,7 @@ export function moduleDef(providers: NgModuleProviderDef[]): NgModuleDefinition
     providersByKey,
     providers,
     modules,
+    isRoot,
   };
 }
 
@@ -119,8 +125,13 @@ export function resolveNgModuleDep(
   return data._parent.get(depDef.token, notFoundValue);
 }
 
+function moduleTransitivelyPresent(ngModule: NgModuleData, scope: any): boolean {
+  return ngModule._def.modules.indexOf(scope) > -1;
+}
+
 function targetsModule(ngModule: NgModuleData, def: InjectableDef): boolean {
-  return def.scope != null && ngModule._def.modules.indexOf(def.scope) > -1;
+  return def.scope != null && (moduleTransitivelyPresent(ngModule, def.scope) ||
+                               def.scope === APP_ROOT_SCOPE && ngModule._def.isRoot);
 }
 
 function _createProviderInstance(ngModule: NgModuleData, providerDef: NgModuleProviderDef): any {

packages/core/src/view/types.ts

@@ -43,6 +43,7 @@ export interface NgModuleDefinition extends Definition<NgModuleDefinitionFactory
   providers: NgModuleProviderDef[];
   providersByKey: {[tokenKey: string]: NgModuleProviderDef};
   modules: any[];
+  isRoot: boolean;
 }
 
 export interface NgModuleDefinitionFactory extends DefinitionFactory<NgModuleDefinition> {}