feat(material/datepicker): Add aria-current="date" to the current date's cell

Author: ByzantineFailure

src/material/datepicker/calendar-body.html

@@ -49,6 +49,7 @@
       [attr.aria-label]="item.ariaLabel"
       [attr.aria-disabled]="!item.enabled || null"
       [attr.aria-selected]="_isSelected(item.compareValue)"
+      [attr.aria-current]="todayValue === item.compareValue ? 'date' : null"
       (click)="_cellClicked(item, $event)"
       [style.width]="_cellWidth"
       [style.paddingTop]="_cellPadding"

src/material/datepicker/calendar-body.spec.ts

@@ -47,9 +47,43 @@ describe('MatCalendarBody', () => {
     });
 
     it('highlights today', () => {
-      const todayCell = calendarBodyNativeElement.querySelector('.mat-calendar-body-today')!;
+      const todayCells = calendarBodyNativeElement.querySelectorAll(
+              '.mat-calendar-body-today')!;
+      expect(todayCells.length).toBe(1);
+
+      const todayCell = todayCells[0];
+
+      expect(todayCell).not.toBeNull();
+      expect(todayCell.textContent!.trim()).toBe('3');
+    });
+
+    it('sets aria-current="date" on today', () => {
+      const todayCells = calendarBodyNativeElement.querySelectorAll(
+              '[aria-current="date"] .mat-calendar-body-today')!;
+      expect(todayCells.length).toBe(1);
+
+      const todayCell = todayCells[0];
+
       expect(todayCell).not.toBeNull();
-      expect(todayCell.innerHTML.trim()).toBe('3');
+      expect(todayCell.textContent!.trim()).toBe('3');
+    });
+
+    it('does not highlight today if today is not within the scope', () => {
+        testComponent.todayValue = 100000;
+        fixture.detectChanges();
+
+        const todayCell = calendarBodyNativeElement.querySelector('.mat-calendar-body-today')!;
+        expect(todayCell).toBeNull();
+    });
+
+    it('does not set aria-current="date" on any cell if today is not ' +
+       'the scope', () => {
+        testComponent.todayValue = 100000;
+        fixture.detectChanges();
+
+        const todayCell = calendarBodyNativeElement.querySelector(
+              '[aria-current="date"] .mat-calendar-body-today')!;
+        expect(todayCell).toBeNull();
     });
 
     it('highlights selected', () => {