fix(forms): number input fires valueChanges twice. (#36087)

Prior to this commit, number input fields would to fire valueChanges twice: once for `input` events when typing and second for the `change` event when the field lost focus (both events happen at once when using the increment and decrement buttons on the number field).

Fixes #12540

BREAKING CHANGE: Number inputs no longer listen to the `change` event.
* Tests which trigger `change` events need to be updated to trigger `input` events instead.
* The `change` event was in place to support IE9, as we found that `input` events were not fired with backspace or cut actions. If you need to maintain IE9 support, you will need to add a change event listener to number inputs and call the `onChange` method of `NumberValueAccessor` manually.
* Lastly, old versions of WebDriver would synthetically trigger the `change` event on `WebElement.clear` and `WebElement.sendKeys`. If you are using an old version of WebDriver, you may need to update tests to ensure `input` events are triggered. For example, you could use `element.sendKeys(Keys.chord(Keys.CONTROL, "a"), Keys.BACK_SPACE);` in place of `element.clear()`.

PR Close #12540

PR Close #36087

packages/forms/src/directives/number_value_accessor.ts

@@ -43,11 +43,7 @@ export const NUMBER_VALUE_ACCESSOR: any = {
 @Directive({
   selector:
       'input[type=number][formControlName],input[type=number][formControl],input[type=number][ngModel]',
-  host: {
-    '(change)': 'onChange($event.target.value)',
-    '(input)': 'onChange($event.target.value)',
-    '(blur)': 'onTouched()'
-  },
+  host: {'(input)': 'onChange($event.target.value)', '(blur)': 'onTouched()'},
   providers: [NUMBER_VALUE_ACCESSOR]
 })
 export class NumberValueAccessor implements ControlValueAccessor {

packages/forms/test/value_accessor_integration_spec.ts

@@ -149,6 +149,23 @@ import {dispatchEvent} from '@angular/platform-browser/testing/src/browser_util'
         expect(control.value).toEqual(0);
       });
 
+      it('should ignore the change event', () => {
+        const fixture = initTest(FormControlNumberInput);
+        const control = new FormControl();
+        fixture.componentInstance.control = control;
+        fixture.detectChanges();
+
+        control.valueChanges.subscribe({
+          next: (value) => {
+            throw 'Input[number] should not react to change event';
+          }
+        });
+        const input = fixture.debugElement.query(By.css('input'));
+
+        input.nativeElement.value = '5';
+        dispatchEvent(input.nativeElement, 'change');
+      });
+
       it('when value is cleared programmatically', () => {
         const fixture = initTest(FormControlNumberInput);
         const control = new FormControl(10);