fix(router): ensure that a route module is only loaded once

Store an observable on the route for the currently outstanding module
load factory.

The replaces the fix angular#26557 as that allowed overlapping requests to still
load the module twice.

Fixes angular#26557 angular#22842

packages/router/src/config.ts

@@ -486,6 +486,12 @@ export interface Route {
    * @internal
    */
   _loadedConfig?: LoadedRouterConfig;
+    /**
+   * Filled for routes with `loadChildren` during load
+   * @internal
+   */
+  _loader$?: Observable<LoadedRouterConfig>;
+
 }
 
 export class LoadedRouterConfig {

packages/router/src/router_config_loader.ts

@@ -8,7 +8,7 @@
 
 import {Compiler, InjectionToken, Injector, NgModuleFactory, NgModuleFactoryLoader} from '@angular/core';
 import {from, Observable, of} from 'rxjs';
-import {map, mergeMap} from 'rxjs/operators';
+import {map, mergeMap, tap, share} from 'rxjs/operators';
 
 import {LoadChildren, LoadedRouterConfig, Route, standardizeConfig} from './config';
 import {flatten, wrapIntoObservable} from './utils/collection';
@@ -27,22 +27,28 @@ export class RouterConfigLoader {
       private onLoadEndListener?: (r: Route) => void) {}
 
   load(parentInjector: Injector, route: Route): Observable<LoadedRouterConfig> {
-    if (this.onLoadStartListener) {
-      this.onLoadStartListener(route);
-    }
+    if (!route._loader$) {
+      if (this.onLoadStartListener) {
+        this.onLoadStartListener(route);
+      }
 
-    const moduleFactory$ = this.loadModuleFactory(route.loadChildren!);
+      const moduleFactory$ = this.loadModuleFactory(route.loadChildren!);
 
-    return moduleFactory$.pipe(map((factory: NgModuleFactory<any>) => {
-      if (this.onLoadEndListener) {
-        this.onLoadEndListener(route);
-      }
+      route._loader$ = moduleFactory$.pipe(
+        map((factory: NgModuleFactory<any>) => {
+          if (this.onLoadEndListener) {
+            this.onLoadEndListener(route);
+          }
 
-      const module = factory.create(parentInjector);
+          const module = factory.create(parentInjector);
 
-      return new LoadedRouterConfig(
-          flatten(module.injector.get(ROUTES)).map(standardizeConfig), module);
-    }));
+          return new LoadedRouterConfig(
+              flatten(module.injector.get(ROUTES)).map(standardizeConfig), module);
+        }),
+        share()
+      );
+    }
+    return route._loader$;
   }
 
   private loadModuleFactory(loadChildren: LoadChildren): Observable<NgModuleFactory<any>> {