refactor(@angular-devkit/build-angular): use ES6 imports instead of require

Main benefit is to use type definitions.

Author: alan-agius4

packages/angular_devkit/build_angular/src/dev-server/index.ts

@@ -14,7 +14,7 @@ import {
 } from '@angular-devkit/build-webpack';
 import { json, tags } from '@angular-devkit/core';
 import * as path from 'path';
-import { Observable, from, of } from 'rxjs';
+import { Observable, from } from 'rxjs';
 import { concatMap, switchMap } from 'rxjs/operators';
 import * as ts from 'typescript';
 import * as url from 'url';
@@ -307,7 +307,7 @@ export function serveWebpackBrowser(
           webpackDevServerFactory: require('webpack-dev-server') as typeof webpackDevServer,
         },
       ).pipe(
-        concatMap((buildEvent, index) => {
+        concatMap(async (buildEvent, index) => {
           // Resolve serve address.
           const serverAddress = url.format({
             protocol: options.ssl ? 'https' : 'http',
@@ -325,16 +325,16 @@ export function serveWebpackBrowser(
             ` + '\n');
 
             if (options.open) {
-              const open = require('open');
-              open(serverAddress);
+              const open = await import('open');
+              await open(serverAddress);
             }
           }
 
           if (buildEvent.success) {
             logger.info(`\n${colors.greenBright(colors.symbols.check)} Compiled successfully.`);
           }
 
-          return of({ ...buildEvent, baseUrl: serverAddress } as DevServerBuilderOutput);
+          return { ...buildEvent, baseUrl: serverAddress } as DevServerBuilderOutput;
         }),
       );
     }),