fix(common): support numeric value as discrete cases for NgPlural (#13876)

PR Close #13876

modules/@angular/common/src/directives/ng_plural.ts

@@ -103,6 +103,7 @@ export class NgPluralCase {
   constructor(
       @Attribute('ngPluralCase') public value: string, template: TemplateRef<Object>,
       viewContainer: ViewContainerRef, @Host() ngPlural: NgPlural) {
-    ngPlural.addCase(value, new SwitchView(viewContainer, template));
+    const isANumber: boolean = !isNaN(Number(value));
+    ngPlural.addCase(isANumber ? `=${value}` : value, new SwitchView(viewContainer, template));
   }
 }

modules/@angular/common/test/directives/ng_plural_spec.ts

@@ -12,7 +12,7 @@ import {ComponentFixture, TestBed, async} from '@angular/core/testing';
 import {expect} from '@angular/platform-browser/testing/matchers';
 
 export function main() {
-  describe('switch', () => {
+  describe('ngPlural', () => {
     let fixture: ComponentFixture<any>;
 
     function getComponent(): TestComponent { return fixture.componentInstance; }
@@ -47,6 +47,22 @@ export function main() {
          detectChangesAndExpectText('you have one message.');
        }));
 
+    it('should display the template according to the exact numeric value', async(() => {
+         const template = '<div>' +
+             '<ul [ngPlural]="switchValue">' +
+             '<template ngPluralCase="0"><li>you have no messages.</li></template>' +
+             '<template ngPluralCase="1"><li>you have one message.</li></template>' +
+             '</ul></div>';
+
+         fixture = createTestComponent(template);
+
+         getComponent().switchValue = 0;
+         detectChangesAndExpectText('you have no messages.');
+
+         getComponent().switchValue = 1;
+         detectChangesAndExpectText('you have one message.');
+       }));
+
     // https://github.com/angular/angular/issues/9868
     // https://github.com/angular/angular/issues/9882
     it('should not throw when ngPluralCase contains expressions', async(() => {