refactor(cdk/portal): simplify DomPortalOutlet constructor (#24504)

Most of the constructor parameters of `DomPortalOutlet` are only used for component portals. These changes make them optional in order to simplify the constructor. If we hit a code path that does require them, we'll throw a runtime error.

Related to the discussion in #24334.

Author: crisbeto

src/cdk/portal/dom-portal-outlet.ts

@@ -22,12 +22,23 @@ import {BasePortalOutlet, ComponentPortal, DomPortal, TemplatePortal} from './po
 export class DomPortalOutlet extends BasePortalOutlet {
   private _document: Document;
 
+  /**
+   * @param outletElement Element into which the content is projected.
+   * @param _componentFactoryResolver Used to resolve the component factory.
+   *   Only required when attaching component portals.
+   * @param _appRef Reference to the application. Only used in component portals when there
+   *   is no `ViewContainerRef` available.
+   * @param _defaultInjector Injector to use as a fallback when the portal being attached doesn't
+   *   have one. Only used for component portals.
+   * @param _document Reference to the document. Used when attaching a DOM portal. Will eventually
+   *   become a required parameter.
+   */
   constructor(
     /** Element into which the content is projected. */
     public outletElement: Element,
-    private _componentFactoryResolver: ComponentFactoryResolver,
-    private _appRef: ApplicationRef,
-    private _defaultInjector: Injector,
+    private _componentFactoryResolver?: ComponentFactoryResolver,
+    private _appRef?: ApplicationRef,
+    private _defaultInjector?: Injector,
 
     /**
      * @deprecated `_document` Parameter to be made required.
@@ -45,7 +56,12 @@ export class DomPortalOutlet extends BasePortalOutlet {
    * @returns Reference to the created component.
    */
   attachComponentPortal<T>(portal: ComponentPortal<T>): ComponentRef<T> {
-    const resolver = portal.componentFactoryResolver || this._componentFactoryResolver;
+    const resolver = (portal.componentFactoryResolver || this._componentFactoryResolver)!;
+
+    if ((typeof ngDevMode === 'undefined' || ngDevMode) && !resolver) {
+      throw Error('Cannot attach component portal to outlet without a ComponentFactoryResolver.');
+    }
+
     const componentFactory = resolver.resolveComponentFactory(portal.component);
     let componentRef: ComponentRef<T>;
 
@@ -62,10 +78,16 @@ export class DomPortalOutlet extends BasePortalOutlet {
 
       this.setDisposeFn(() => componentRef.destroy());
     } else {
-      componentRef = componentFactory.create(portal.injector || this._defaultInjector);
-      this._appRef.attachView(componentRef.hostView);
+      if ((typeof ngDevMode === 'undefined' || ngDevMode) && !this._appRef) {
+        throw Error('Cannot attach component portal to outlet without an ApplicationRef.');
+      }
+
+      componentRef = componentFactory.create(
+        portal.injector || this._defaultInjector || Injector.NULL,
+      );
+      this._appRef!.attachView(componentRef.hostView);
       this.setDisposeFn(() => {
-        this._appRef.detachView(componentRef.hostView);
+        this._appRef!.detachView(componentRef.hostView);
         componentRef.destroy();
       });
     }

tools/public_api_guard/cdk/portal.md

@@ -104,7 +104,7 @@ export class DomPortalHost extends DomPortalOutlet {
 // @public
 export class DomPortalOutlet extends BasePortalOutlet {
     constructor(
-    outletElement: Element, _componentFactoryResolver: ComponentFactoryResolver, _appRef: ApplicationRef, _defaultInjector: Injector,
+    outletElement: Element, _componentFactoryResolver?: ComponentFactoryResolver | undefined, _appRef?: ApplicationRef | undefined, _defaultInjector?: Injector | undefined,
     _document?: any);
     attachComponentPortal<T>(portal: ComponentPortal<T>): ComponentRef<T>;
     // @deprecated