refactor(router): cleanup to navigation stream for readability and documentation (#25740)

* Pull out `activateRoutes` into new operator
* Add `asyncTap` operator
* Use `asyncTap` operator for router hooks and remove corresponding abstracted operators
* Clean up formatting
* Minor performance improvements

PR Close #25740

packages/router/src/operators/switch_tap.ts

@@ -0,0 +1,29 @@
+/**
+ * @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 {MonoTypeOperatorFunction, ObservableInput, from} from 'rxjs';
+import {map, switchMap} from 'rxjs/operators';
+
+/**
+ * Perform a side effect through a switchMap for every emission on the source Observable,
+ * but return an Observable that is identical to the source. It's essentially the same as
+ * the `tap` operator, but if the side effectful `next` function returns an ObservableInput,
+ * it will wait before continuing with the original value.
+ */
+export function switchTap<T>(next: (x: T) => void|ObservableInput<any>):
+    MonoTypeOperatorFunction<T> {
+  return function(source) {
+    return source.pipe(switchMap(v => {
+      const nextResult = next(v);
+      if (nextResult) {
+        return from(nextResult).pipe(map(() => v));
+      }
+      return from([v]);
+    }));
+  };
+}