fix(menu): toggle aria-expanded attribute on menu trigger (#11751)

[Based on the accessibility guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/#menubutton), buttons that trigger a menu should have the `aria-expanded` attribute.

src/lib/menu/menu-trigger.ts

@@ -70,6 +70,7 @@ export const MENU_PANEL_TOP_PADDING = 8;
   selector: `[mat-menu-trigger-for], [matMenuTriggerFor]`,
   host: {
     'aria-haspopup': 'true',
+    '[attr.aria-expanded]': 'menuOpen || null',
     '(mousedown)': '_handleMousedown($event)',
     '(keydown)': '_handleKeydown($event)',
     '(click)': '_handleClick($event)',

src/lib/menu/menu.spec.ts

@@ -388,6 +388,24 @@ describe('MatMenu', () => {
       expect(items[2].classList).toContain('cdk-keyboard-focused');
     }));
 
+  it('should toggle the aria-expanded attribute on the trigger', () => {
+    const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
+    fixture.detectChanges();
+    const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
+
+    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+
+    fixture.componentInstance.trigger.openMenu();
+    fixture.detectChanges();
+
+    expect(triggerEl.getAttribute('aria-expanded')).toBe('true');
+
+    fixture.componentInstance.trigger.closeMenu();
+    fixture.detectChanges();
+
+    expect(triggerEl.hasAttribute('aria-expanded')).toBe(false);
+  });
+
   describe('lazy rendering', () => {
     it('should be able to render the menu content lazily', fakeAsync(() => {
       const fixture = createComponent(SimpleLazyMenu);