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

@@ -50,11 +50,45 @@ 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.innerHTML.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');
     });
 
+    it('does not highlight today if today is not within the scope', () => {
+        testComponent.todayValue = null;
+        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 = null;
+        fixture.detectChanges();
+
+        const todayCell = calendarBodyNativeElement.querySelector(
+              '[aria-current="date"] .mat-calendar-body-today')!;
+        expect(todayCell).toBeNull();
+    });
+
     it('highlights selected', () => {
       const selectedCell = calendarBodyNativeElement.querySelector('.mat-calendar-body-selected')!;
       expect(selectedCell).not.toBeNull();
@@ -595,7 +629,7 @@ describe('MatCalendarBody', () => {
     <table mat-calendar-body
           [label]="label"
           [rows]="rows"
-          [todayValue]="todayValue"
+          [todayValue]="todayValue!"
           [startValue]="selectedValue"
           [endValue]="selectedValue"
           [labelMinRequiredCells]="labelMinRequiredCells"
@@ -607,7 +641,7 @@ describe('MatCalendarBody', () => {
 class StandardCalendarBody {
   label = 'Jan 2017';
   rows = createCalendarCells(2);
-  todayValue = 3;
+  todayValue: number|null = 3;
   selectedValue = 4;
   labelMinRequiredCells = 3;
   numCols = 7;