fix(slide-toggle): redirect focus to underlying input element

Currently the `mat-slide-toggle` doesn't redirect focus to the underlying `input` element. This means that things like the focus trap's `cdkFocusInitial` won't work when they're set on the toggle host. These changes add a `tabindex` and set up a `focus` listener to forward focus to the input.

Relates to angular#13953.

src/lib/slide-toggle/slide-toggle.spec.ts

@@ -293,6 +293,15 @@ describe('MatSlideToggle without forms', () => {
       expect(document.activeElement).toBe(inputElement);
     });
 
+    it('should focus on underlying input element when the host is focused', () => {
+      expect(document.activeElement).not.toBe(inputElement);
+
+      slideToggleElement.focus();
+      fixture.detectChanges();
+
+      expect(document.activeElement).toBe(inputElement);
+    });
+
     it('should set a element class if labelPosition is set to before', () => {
       expect(slideToggleElement.classList).not.toContain('mat-slide-toggle-label-before');
 
@@ -355,7 +364,7 @@ describe('MatSlideToggle without forms', () => {
       fixture.detectChanges();
 
       const slideToggle = fixture.debugElement.query(By.directive(MatSlideToggle)).nativeElement;
-      expect(slideToggle.getAttribute('tabindex')).toBeFalsy();
+      expect(slideToggle.getAttribute('tabindex')).toBe('-1');
     }));
   });
 

src/lib/slide-toggle/slide-toggle.ts

@@ -86,11 +86,12 @@ export const _MatSlideToggleMixinBase:
   host: {
     'class': 'mat-slide-toggle',
     '[id]': 'id',
-    '[attr.tabindex]': 'null',
+    '[attr.tabindex]': '-1', // Needs to be `-1` so it can still receive programmatic focus.
     '[class.mat-checked]': 'checked',
     '[class.mat-disabled]': 'disabled',
     '[class.mat-slide-toggle-label-before]': 'labelPosition == "before"',
     '[class._mat-animation-noopable]': '_animationMode === "NoopAnimations"',
+    '(focus)': '_inputElement.nativeElement.focus()',
   },
   templateUrl: 'slide-toggle.html',
   styleUrls: ['slide-toggle.css'],