chore: fix docs app console errors

docs_app/src/app/custom-elements/code/pretty-printer.service.ts

@@ -5,10 +5,6 @@ import { first, map, share } from 'rxjs/operators';
 
 import { Logger } from 'app/shared/logger.service';
 
-declare const System: {
-  import(name: string): Promise<any>;
-};
-
 type PrettyPrintOne = (code: string, language?: string, linenums?: number | boolean) => string;
 
 /**
@@ -24,12 +20,13 @@ export class PrettyPrinter {
   }
 
   private getPrettyPrintOne(): Promise<PrettyPrintOne> {
-    const ppo = (window as any)['prettyPrintOne'];
+    const ppo = (window as any).prettyPrintOne;
     return ppo ? Promise.resolve(ppo) :
-      // prettify.js is not in window global; load it with webpack loader
-      System.import('assets/js/prettify.js')
+      // `prettyPrintOne` is not on `window`, which means `prettify.js` has not been loaded yet.
+      // Import it; as a side-effect it will add `prettyPrintOne` on `window`.
+      import('assets/js/prettify.js' as any)
         .then(
-          () => (window as any)['prettyPrintOne'],
+          () => (window as any).prettyPrintOne,
           err => {
             const msg = `Cannot get prettify.js from server: ${err.message}`;
             this.logger.error(new Error(msg));