title: Where does Angular define "as local-var" behavior for *ngIf?
body: <p>I am trying to understand where is the "as local-var" optional behavior of <em>ngIf</em> defined, e.g.:
  <code>*ngIf="user$ | async as user"</code></p>
  
  <p>Tried looking into obvious places in source, like
  <a href="https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_if.ts" rel="noreferrer">https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_if.ts</a></p>
  
  <p>But nothing in the code, only docs.</p>
  
  <p>Does anyone know where in the code this magic is happening ?</p>