message: docs: add section on binding to passive events
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  PR Close
diff: @@ -24,6 +24,26 @@ The event binding listens for the button's click events and calls the component'
  <img src='generated/images/guide/template-syntax/syntax-diagram.svg' alt="Syntax diagram">
  </div>
  +## Binding to passive events
  +
  +Angular also supports passive event listeners. For example, you can use the following steps to make a scroll event passive.
  +
  +1. Create a file `zone-flags.ts` under `src` directory.
  +2. Add the following line into this file.
  +
  +```
  +(window as any)['__zone_symbol__PASSIVE_EVENTS'] = ['scroll'];
  +```
  +
  +3. In the `src/polyfills.ts` file, before importing zone.js, import the newly created `zone-flags`.
  +
  +```
  +import './zone-flags';
  +import 'zone.js'; // Included with Angular CLI.
  +```
  +
  +After those steps, if you add event listeners for the `scroll` event, the listeners will be `passive`.
  +
  ## Custom events with `EventEmitter`
  [Directives](guide/built-in-directives) typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter) as follows.
  