fix: add aria-disabled to disabled elements

Author: calebdwilliams

Diff for: src/utils.ts

@@ -10,6 +10,11 @@ const observer = new MutationObserver((mutationsList: MutationRecord[]) => {
     if (target.constructor['formAssociated']) {
       const isDisabled = target.hasAttribute('disabled');
       target.toggleAttribute('internals-disabled', isDisabled);
+      if (isDisabled) {
+        target.setAttribute('aria-disabled', 'true');
+      } else {
+        target.removeAttribute('aria-disabled');
+      }
       if (target.formDisabledCallback) {
         target.formDisabledCallback.apply(target, [target.hasAttribute('disabled')]);
       }

Diff for: test/ElementInternals.test.js

@@ -314,6 +314,9 @@ describe('The ElementInternals polyfill', () => {
       el.disabled = true;
       await elementUpdated(el);
       expect(internals.willValidate).to.be.false;
+      if (ElementInternals.isPolyfilled) {
+        expect(el.getAttribute('aria-disabled')).to.equal('true');
+      }
     });
 
     it('will participate in forms', async () => {