NgIf syntax has been extended to support else clause to display template
when the condition is false. In addition the condition value can now
be stored in local variable, for later reuse. This is especially useful
when used with the `async` pipe.
Example:
```
<div *ngIf="userObservable | async; else loading; let user">
Hello {{user.last}}, {{user.first}}!
</div>
<template #loading>Waiting...</template>
```
closes#13061closes#13297
Ryan Cavanaugh
Victor Berchet
Misko Hevery
Chuck Jazdzewski
vsavkin