fix(@schematics/angular): import zone related flags in other files

fix #12886

packages/schematics/angular/application/files/src/polyfills.ts

@@ -57,18 +57,27 @@
 /**
  * By default, zone.js will patch all possible macroTask and DomEvents
  * user can disable parts of macroTask/DomEvents patch by setting following flags
+ * because those flags need to be set before `zone.js` being loaded, and webpack
+ * will put import in the top of bundle, so user need to create a separate file
+ * in this directory (for example: zone-flags.ts), and put the following flags
+ * into that file, and then add the following code before importing zone.js.
+ * import './zone-flags.ts';
+ *
+ * The flags allowed in zone-flags.ts are listed here.
+ *
+ * The following flags will work for all browsers.
+ *
+ * (window as any).__Zone_disable_requestAnimationFrame = true; // disable patch requestAnimationFrame
+ * (window as any).__Zone_disable_on_property = true; // disable patch onProperty such as onclick
+ * (window as any).__zone_symbol__BLACK_LISTED_EVENTS = ['scroll', 'mousemove']; // disable patch specified eventNames
+ *
+ *  in IE/Edge developer tools, the addEventListener will also be wrapped by zone.js
+ *  with the following flag, it will bypass `zone.js` patch for IE/Edge
+ *
+ *  (window as any).__Zone_enable_cross_context_check = true;
+ *
  */
 
- // (window as any).__Zone_disable_requestAnimationFrame = true; // disable patch requestAnimationFrame
- // (window as any).__Zone_disable_on_property = true; // disable patch onProperty such as onclick
- // (window as any).__zone_symbol__BLACK_LISTED_EVENTS = ['scroll', 'mousemove']; // disable patch specified eventNames
-
- /*
- * in IE/Edge developer tools, the addEventListener will also be wrapped by zone.js
- * with the following flag, it will bypass `zone.js` patch for IE/Edge
- */
-// (window as any).__Zone_enable_cross_context_check = true;
-
 /***************************************************************************************************
  * Zone JS is required by default for Angular itself.
  */