fix(@schematics/schematics): Make sure promise returning methods are async

Method returning Promise should be async
Rationale: https://palantir.github.io/tslint/rules/promise-function-async/

Author: apeeters

integration/angular_cli/e2e/src/app.po.ts

@@ -1,11 +1,11 @@
 import { browser, by, element } from 'protractor';
 
 export class AppPage {
-  navigateTo(): Promise<unknown> {
+  async navigateTo(): Promise<unknown> {
     return browser.get(browser.baseUrl) as Promise<unknown>;
   }
 
-  getTitleText(): Promise<string> {
+  async getTitleText(): Promise<string> {
     return element(by.css('app-root .content span')).getText() as Promise<string>;
   }
 }

packages/angular_devkit/build_angular/test/hello-world-app/e2e/app.po.ts

@@ -8,11 +8,11 @@
 import { browser, by, element } from 'protractor';
 
 export class AppPage {
-  navigateTo(): Promise<unknown> {
+  async navigateTo(): Promise<unknown> {
     return browser.get(browser.baseUrl) as Promise<unknown>;
   }
 
-  getTitleText(): Promise<string> {
+  async getTitleText(): Promise<string> {
     return element(by.css('app-root h1')).getText() as Promise<string>;
   }
 }

packages/schematics/angular/e2e/files/src/app.po.ts.template

@@ -1,11 +1,11 @@
 import { browser, by, element } from 'protractor';
 
 export class AppPage {
-  navigateTo(): Promise<unknown> {
+  async navigateTo(): Promise<unknown> {
     return browser.get(browser.baseUrl) as Promise<unknown>;
   }
 
-  getTitleText(): Promise<string> {
+  async getTitleText(): Promise<string> {
     return element(by.css('<%= rootSelector %> .content span')).getText() as Promise<string>;
   }
 }