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/architecture/e2e/src/app.e2e-spec.ts

@@ -19,7 +19,7 @@ describe('Architecture', () => {
   });
 
   it(`has h2 '${expectedH2}'`, () => {
-    let h2 = element.all(by.css('h2')).map((elt: any) => elt.getText());
+    const h2 = element.all(by.css('h2')).map((elt: any) => elt.getText());
     expect(h2).toEqual(expectedH2);
   });
 
@@ -32,29 +32,29 @@ function heroTests() {
   const targetHero: Hero = { id: 2, name: 'Dr Nice' };
 
   it('has the right number of heroes', () => {
-    let page = getPageElts();
+    const page = getPageElts();
     expect(page.heroes.count()).toEqual(3);
   });
 
   it('has no hero details initially', () => {
-    let page = getPageElts();
+    const page = getPageElts();
     expect(page.heroDetail.isPresent()).toBeFalsy('no hero detail');
   });
 
   it('shows selected hero details', async () => {
     await element(by.cssContainingText('li', targetHero.name)).click();
-    let page = getPageElts();
-    let hero = await heroFromDetail(page.heroDetail);
+    const page = getPageElts();
+    const hero = await heroFromDetail(page.heroDetail);
     expect(hero.id).toEqual(targetHero.id);
     expect(hero.name).toEqual(targetHero.name);
   });
 
   it(`shows updated hero name in details`, async () => {
-    let input = element.all(by.css('input')).first();
+    const input = element.all(by.css('input')).first();
     input.sendKeys(nameSuffix);
-    let page = getPageElts();
-    let hero = await heroFromDetail(page.heroDetail);
-    let newName = targetHero.name + nameSuffix;
+    const page = getPageElts();
+    const hero = await heroFromDetail(page.heroDetail);
+    const newName = targetHero.name + nameSuffix;
     expect(hero.id).toEqual(targetHero.id);
     expect(hero.name).toEqual(newName);
   });
@@ -62,12 +62,12 @@ function heroTests() {
 
 function salesTaxTests() {
   it('has no sales tax initially', () => {
-    let page = getPageElts();
+    const page = getPageElts();
     expect(page.salesTaxDetail.isPresent()).toBeFalsy('no sales tax info');
   });
 
   it('shows sales tax', async () => {
-    let page = getPageElts();
+    const page = getPageElts();
     page.salesTaxAmountInput.sendKeys('10', protractor.Key.ENTER);
     expect(page.salesTaxDetail.getText()).toEqual('The sales tax is $1.00');
   });
@@ -86,9 +86,9 @@ function getPageElts() {
 
 async function heroFromDetail(detail: ElementFinder): Promise<Hero> {
   // Get hero id from the first <div>
-  let id = await detail.all(by.css('div')).first().getText();
+  const id = await detail.all(by.css('div')).first().getText();
   // Get name from the h2
-  let name = await detail.element(by.css('h4')).getText();
+  const name = await detail.element(by.css('h4')).getText();
   return {
     id: +id.substr(id.indexOf(' ') + 1),
     name: name.substr(0, name.lastIndexOf(' ')),

aio/content/examples/architecture/src/app/backend.service.ts

@@ -18,7 +18,7 @@ export class BackendService {
       // TODO: get from the database
       return Promise.resolve<Hero[]>(HEROES);
     }
-    let err = new Error('Cannot get object of this type');
+    const err = new Error('Cannot get object of this type');
     this.logger.error(err);
     throw err;
   }

aio/content/examples/architecture/src/app/sales-tax.service.ts

@@ -7,7 +7,7 @@ export class SalesTaxService {
   constructor(private rateService: TaxRateService) { }
 
   getVAT(value: string | number) {
-    let amount = (typeof value === 'string') ?
+    const amount = (typeof value === 'string') ?
       parseFloat(value) : value;
     return (amount || 0) * this.rateService.getRate('VAT');
   }