fix(input): set proper role on md-error (#6259)

Sets the `alert` role on `md-error` instances, causing the messages to be read out properly by screen readers. For reference: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_alert_role

Author: crisbeto

src/lib/input/input-container.spec.ts

@@ -814,6 +814,13 @@ describe('MdInputContainer with forms', () => {
       });
     }));
 
+    it('should set the proper role on the error messages', () => {
+      testComponent.formControl.markAsTouched();
+      fixture.detectChanges();
+
+      expect(containerEl.querySelector('md-error')!.getAttribute('role')).toBe('alert');
+    });
+
     it('sets the aria-describedby to reference errors when in error state', () => {
       let hintId = fixture.debugElement.query(By.css('.mat-hint')).nativeElement.getAttribute('id');
       let describedBy = inputEl.getAttribute('aria-describedby');

src/lib/input/input-container.ts

@@ -102,6 +102,7 @@ export class MdHint {
   selector: 'md-error, mat-error',
   host: {
     'class': 'mat-input-error',
+    'role': 'alert',
     '[attr.id]': 'id',
   }
 })