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
BREAKING CHANGE:
The deprecated forms APIs in @angular/common have been removed. Please update to the new forms API in @angular/forms. See angular.io for more information.
