fix(datepicker): able to open from readonly input using keyboard control (#12880)

Fixes users being able to open a datepicker through a `readonly` input by using the alt + down arrow key combination.

src/lib/datepicker/datepicker-input.ts

@@ -311,7 +311,9 @@ export class MatDatepickerInput<D> implements ControlValueAccessor, OnDestroy, V
   }
 
   _onKeydown(event: KeyboardEvent) {
-    if (this._datepicker && event.altKey && event.keyCode === DOWN_ARROW) {
+    const isAltDownArrow = event.altKey && event.keyCode === DOWN_ARROW;
+
+    if (this._datepicker && isAltDownArrow && !this._elementRef.nativeElement.readOnly) {
       this._datepicker.open();
       event.preventDefault();
     }

src/lib/datepicker/datepicker.spec.ts

@@ -454,7 +454,7 @@ describe('MatDatepicker', () => {
         expect(testComponent.datepicker.opened).toBe(false);
       }));
 
-      it('should open the datpeicker using ALT + DOWN_ARROW', fakeAsync(() => {
+      it('should open the datepicker using ALT + DOWN_ARROW', fakeAsync(() => {
         expect(testComponent.datepicker.opened).toBe(false);
 
         const event = createKeyboardEvent('keydown', DOWN_ARROW);
@@ -468,6 +468,24 @@ describe('MatDatepicker', () => {
         expect(event.defaultPrevented).toBe(true);
       }));
 
+      it('should not open for ALT + DOWN_ARROW on readonly input', fakeAsync(() => {
+        const input = fixture.nativeElement.querySelector('input');
+
+        expect(testComponent.datepicker.opened).toBe(false);
+
+        input.setAttribute('readonly', 'true');
+
+        const event = createKeyboardEvent('keydown', DOWN_ARROW);
+        Object.defineProperty(event, 'altKey', {get: () => true});
+
+        dispatchEvent(input, event);
+        fixture.detectChanges();
+        flush();
+
+        expect(testComponent.datepicker.opened).toBe(false);
+        expect(event.defaultPrevented).toBe(false);
+      }));
+
     });
 
     describe('datepicker with too many inputs', () => {