fix(material/menu): missing panelClass getter (#33191)

Author: ok7sai

goldens/material/menu/index.api.md

@@ -120,6 +120,7 @@ export class MatMenu implements AfterContentInit, MatMenuPanel<MatMenuItem>, OnI
     overlapTrigger: boolean;
     overlayPanelClass: string | string[];
     _panelAnimationState: 'void' | 'enter';
+    get panelClass(): string;
     set panelClass(classes: string);
     // (undocumented)
     readonly panelId: string;

src/material/menu/menu.spec.ts

@@ -568,6 +568,15 @@ describe('MatMenu', () => {
     expect(panel.classList).toContain('custom-two');
   });
 
+  it('should expose the configured classes via the getter', () => {
+    const fixture = TestBed.createComponent(SimpleMenu);
+    fixture.componentInstance.panelClass = 'custom-one custom-two';
+    fixture.detectChanges();
+
+    expect(fixture.componentInstance.menu.panelClass).toBe('custom-one custom-two');
+    expect(fixture.componentInstance.menu.classList).toBe('custom-one custom-two');
+  });
+
   it('should set the "menu" role on the overlay panel', () => {
     const fixture = TestBed.createComponent(SimpleMenu);
     fixture.detectChanges();

src/material/menu/menu.ts

@@ -221,6 +221,9 @@ export class MatMenu implements AfterContentInit, MatMenuPanel<MatMenuItem>, OnI
    * @param classes list of class names
    */
   @Input('class')
+  get panelClass(): string {
+    return this._previousPanelClass;
+  }
   set panelClass(classes: string) {
     const previousPanelClass = this._previousPanelClass;
     const newClassList = {...this._classList};
@@ -243,7 +246,7 @@ export class MatMenu implements AfterContentInit, MatMenuPanel<MatMenuItem>, OnI
 
     this._classList = newClassList;
   }
-  private _previousPanelClass!: string;
+  private _previousPanelClass: string = '';
 
   /**
    * This method takes classes set on the host mat-menu element and applies them on the