fix(material-experimental/mdc-chips): make aria-hidden element non-focusable

The MDC-based `MatChipRemove` has a default `tabindex` of 0 and inherits `aria-hidden` from `MatChipTrailingIcon` which is invalid and is flagged by axe. These changes switch the default `tabindex` to -1.

Author: crisbeto

src/material-experimental/mdc-chips/chip-icons.ts

@@ -84,7 +84,7 @@ const _MatChipRemoveMixinBase:
   CanDisableCtor &
   HasTabIndexCtor &
   typeof MatChipRemoveBase =
-    mixinTabIndex(mixinDisabled(MatChipRemoveBase));
+    mixinTabIndex(mixinDisabled(MatChipRemoveBase), -1);
 
 /**
  * Directive to remove the parent chip when the trailing icon is clicked or
@@ -95,9 +95,11 @@ const _MatChipRemoveMixinBase:
  *
  * Example:
  *
- *     `<mat-chip>
- *       <mat-icon matChipRemove>cancel</mat-icon>
- *     </mat-chip>`
+ * ```
+ * <mat-chip>
+ *   <mat-icon matChipRemove>cancel</mat-icon>
+ * </mat-chip>
+ * ```
  */
 @Directive({
   selector: '[matChipRemove]',

src/material-experimental/mdc-chips/chip-remove.spec.ts

@@ -78,6 +78,16 @@ describe('MDC-based Chip Remove', () => {
 
       expect(chipNativeElement.classList.contains('mdc-chip--exit')).toBe(false);
     });
+
+    it('should not make the button focusable when it has aria-hidden', () => {
+      const buttonElement = chipNativeElement.querySelector('button')!;
+
+      // If we ever end up removing the `aria-hidden`, we can drop
+      // this test and allow the element to be focusable.
+      expect(buttonElement.getAttribute('aria-hidden')).toBe('true');
+      expect(buttonElement.getAttribute('tabindex')).toBe('-1');
+    });
+
   });
 });