refactor(docs-infra): fix docs examples for tslint rule prefer-const (#38143)

This commit updates the docs examples to be compatible with the
`prefer-const` tslint rule.

This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.

PR Close #38143

aio/content/examples/testing/e2e/src/app.e2e-spec.ts

@@ -6,7 +6,7 @@ describe('Testing Example', () => {
   beforeAll(() => browser.get(''));
 
   function getPageElts() {
-    let navElts = element.all(by.css('app-root nav a'));
+    const navElts = element.all(by.css('app-root nav a'));
 
     return {
       navElts,
@@ -20,13 +20,13 @@ describe('Testing Example', () => {
   });
 
   it(`has views ${expectedViewNames}`, async () => {
-    let viewNames = getPageElts().navElts.map(async (el: ElementFinder) => await el.getText());
+    const viewNames = getPageElts().navElts.map(async (el: ElementFinder) => await el.getText());
 
     expect(viewNames).toEqual(expectedViewNames);
   });
 
   it('has dashboard as the active view', () => {
-    let page = getPageElts();
+    const page = getPageElts();
 
     expect(page.appDashboard.isPresent()).toBeTruthy();
   });

aio/content/examples/testing/src/app/dashboard/dashboard.component.ts

@@ -28,13 +28,13 @@ export class DashboardComponent implements OnInit {
 
   // #docregion goto-detail
   gotoDetail(hero: Hero) {
-    let url = `/heroes/${hero.id}`;
+    const url = `/heroes/${hero.id}`;
     this.router.navigateByUrl(url);
   }
   // #enddocregion goto-detail
 
   get title() {
-    let cnt = this.heroes.length;
+    const cnt = this.heroes.length;
     return cnt === 0 ? 'No Heroes' :
       cnt === 1 ? 'Top Hero' :  `Top ${cnt} Heroes`;
   }

aio/content/examples/testing/src/app/demo/demo.ts

@@ -310,11 +310,11 @@ export class MyIfChildComponent implements OnInit, OnChanges, OnDestroy {
   }
 
   ngOnChanges(changes: {[propertyName: string]: SimpleChange}) {
-    for (let propName in changes) {
+    for (const propName in changes) {
       this.ngOnChangesCounter += 1;
-      let prop = changes[propName];
-      let cur  = JSON.stringify(prop.currentValue);
-      let prev = JSON.stringify(prop.previousValue);
+      const prop = changes[propName];
+      const cur  = JSON.stringify(prop.currentValue);
+      const prev = JSON.stringify(prop.previousValue);
       this.changeLog.push(`${propName}: currentValue = ${cur}, previousValue = ${prev}`);
     }
   }

aio/content/examples/testing/src/app/model/testing/http-client.spec.ts

@@ -89,7 +89,7 @@ describe('HttpClient testing', () => {
   });
 
   it('can test multiple requests', () => {
-    let testData: Data[] = [
+    const testData: Data[] = [
       { name: 'bob' }, { name: 'carol' },
       { name: 'ted' }, { name: 'alice' }
     ];

aio/content/examples/testing/src/app/shared/title-case.pipe.spec.ts

@@ -5,7 +5,7 @@ import { TitleCasePipe } from './title-case.pipe';
 // #docregion excerpt, mini-excerpt
 describe('TitleCasePipe', () => {
   // This pipe is a pure, stateless function so no need for BeforeEach
-  let pipe = new TitleCasePipe();
+  const pipe = new TitleCasePipe();
 
   it('transforms "abc" to "Abc"', () => {
     expect(pipe.transform('abc')).toBe('Abc');

aio/content/examples/testing/src/testing/index.ts

@@ -21,7 +21,7 @@ export function advance(f: ComponentFixture<any>): void {
  * Although officially deprecated, some browsers (phantom) don't accept the preferred "new Event(eventName)"
  */
 export function newEvent(eventName: string, bubbles = false, cancelable = false) {
-  let evt = document.createEvent('CustomEvent');  // MUST be 'CustomEvent'
+  const evt = document.createEvent('CustomEvent');  // MUST be 'CustomEvent'
   evt.initCustomEvent(eventName, bubbles, cancelable, null);
   return evt;
 }