feat(menu): add indicator to menu items that trigger a sub-menu

src/lib/menu/_menu-theme.scss

@@ -18,12 +18,11 @@
     &[disabled] {
       color: mat-color($foreground, 'disabled');
     }
+  }
 
-    .mat-icon {
-      color: mat-color($foreground, 'icon');
-      vertical-align: middle;
-    }
-
+  .mat-menu-item .mat-icon,
+  .mat-menu-item-submenu-trigger::after {
+    color: mat-color($foreground, 'icon');
   }
 
   .mat-menu-item:hover,

src/lib/menu/menu-item.ts

@@ -28,6 +28,7 @@ export const _MdMenuItemMixinBase = mixinDisabled(MdMenuItemBase);
     'role': 'menuitem',
     'class': 'mat-menu-item',
     '[class.mat-menu-item-highlighted]': '_highlighted',
+    '[class.mat-menu-item-submenu-trigger]': '_triggersSubmenu',
     '[attr.tabindex]': '_getTabIndex()',
     '[attr.aria-disabled]': 'disabled.toString()',
     '[attr.disabled]': 'disabled || null',
@@ -45,6 +46,9 @@ export class MdMenuItem extends _MdMenuItemMixinBase implements Focusable, CanDi
   /** Whether the menu item is highlighted. */
   _highlighted: boolean = false;
 
+  /** Whether the menu item acts as a trigger for a sub-menu. */
+  _triggersSubmenu: boolean = false;
+
   constructor(private _elementRef: ElementRef) {
     super();
   }

src/lib/menu/menu-trigger.ts

@@ -122,13 +122,19 @@ export class MdMenuTrigger implements AfterViewInit, OnDestroy {
   /** Event emitted when the associated menu is closed. */
   @Output() onMenuClose = new EventEmitter<void>();
 
-  constructor(private _overlay: Overlay,
-              private _element: ElementRef,
-              private _viewContainerRef: ViewContainerRef,
-              @Inject(MD_MENU_SCROLL_STRATEGY) private _scrollStrategy,
-              @Optional() private _parentMenu: MdMenu,
-              @Optional() @Self() private _menuItemInstance: MdMenuItem,
-              @Optional() private _dir: Directionality) { }
+  constructor(
+    private _overlay: Overlay,
+    private _element: ElementRef,
+    private _viewContainerRef: ViewContainerRef,
+    @Inject(MD_MENU_SCROLL_STRATEGY) private _scrollStrategy,
+    @Optional() private _parentMenu: MdMenu,
+    @Optional() @Self() private _menuItemInstance: MdMenuItem,
+    @Optional() private _dir: Directionality) {
+
+    if (_menuItemInstance) {
+      _menuItemInstance._triggersSubmenu = this.triggersSubmenu();
+    }
+  }
 
   ngAfterViewInit() {
     this._checkMenu();

src/lib/menu/menu.scss

@@ -1,5 +1,4 @@
 // TODO(kara): update vars for desktop when MD team responds
-// TODO(kara): animation for menu opening
 
 @import '../core/style/button-common';
 @import '../core/style/layout-common';
@@ -8,6 +7,7 @@
 
 $mat-menu-vertical-padding: 8px !default;
 $mat-menu-border-radius: 2px !default;
+$mat-menu-submenu-indicator-size: 10px !default;
 
 .mat-menu-panel {
   @include mat-menu-base(2);
@@ -29,6 +29,43 @@ $mat-menu-border-radius: 2px !default;
   @include mat-button-reset();
   @include mat-menu-item-base();
   position: relative;
+
+  .mat-icon {
+    vertical-align: middle;
+  }
+}
+
+.mat-menu-item-submenu-trigger {
+  // Increase the side padding to prevent the indicator from overlapping the text.
+  padding-right: $mat-menu-side-padding * 2;
+
+  // Renders a triangle to indicate that the menu item will open a sub-menu.
+  &::after {
+    $size: $mat-menu-submenu-indicator-size / 2;
+
+    width: 0;
+    height: 0;
+    border-style: solid;
+    border-width: $size 0 $size $size;
+    border-color: transparent transparent transparent currentColor;
+    content: '';
+    display: inline-block;
+    position: absolute;
+    top: 50%;
+    right: $mat-menu-side-padding;
+    transform: translateY(-50%);
+  }
+
+  [dir='rtl'] & {
+    padding-right: $mat-menu-side-padding / 2;
+    padding-left: $mat-menu-side-padding * 2;
+
+    &::after {
+      right: auto;
+      left: $mat-menu-side-padding;
+      transform: rotateY(180deg) translateY(-50%);
+    }
+  }
 }
 
 button.mat-menu-item {

src/lib/menu/menu.spec.ts

@@ -871,6 +871,17 @@ describe('MdMenu', () => {
       expect(overlay.querySelectorAll('.mat-menu-panel').length).toBe(0, 'Expected no open menus');
     });
 
+    it('should set a class on the menu items that trigger a sub-menu', () => {
+      compileTestComponent();
+      instance.rootTrigger.openMenu();
+      fixture.detectChanges();
+
+      const menuItems = overlay.querySelectorAll('[md-menu-item]');
+
+      expect(menuItems[0].classList).toContain('mat-menu-item-submenu-trigger');
+      expect(menuItems[1].classList).not.toContain('mat-menu-item-submenu-trigger');
+    });
+
   });
 
 });