chore: fix compilation error in bottom sheet tests (#13775)

Fixes a compilation error that got in through a PR that was opened before we updated to TS 3.1.

Author: crisbeto

src/lib/bottom-sheet/bottom-sheet.spec.ts

@@ -582,18 +582,18 @@ describe('MatBottomSheet', () => {
       viewContainerFixture.detectChanges();
       flushMicrotasks();
 
-      expect(document.activeElement.id)
+      expect(document.activeElement!.id)
           .not.toBe('bottom-sheet-trigger', 'Expected the focus to change when sheet was opened.');
 
       bottomSheetRef.dismiss();
-      expect(document.activeElement.id).not.toBe('bottom-sheet-trigger',
+      expect(document.activeElement!.id).not.toBe('bottom-sheet-trigger',
           'Expcted the focus not to have changed before the animation finishes.');
 
       flushMicrotasks();
       viewContainerFixture.detectChanges();
       tick(500);
 
-      expect(document.activeElement.id).not.toBe('bottom-sheet-trigger',
+      expect(document.activeElement!.id).not.toBe('bottom-sheet-trigger',
           'Expected the trigger not to be refocused on close.');
 
       document.body.removeChild(button);