chore: add missing mat prefix selectors (#7114)

Author: mmalerba

src/lib/menu/menu-directive.ts

@@ -66,7 +66,7 @@ const MD_MENU_BASE_ELEVATION = 2;
     transformMenu,
     fadeInItems
   ],
-  exportAs: 'mdMenu'
+  exportAs: 'mdMenu, matMenu'
 })
 export class MdMenu implements AfterContentInit, MdMenuPanel, OnDestroy {
   private _keyManager: FocusKeyManager<MdMenuItem>;

src/lib/menu/menu-trigger.ts

@@ -81,7 +81,7 @@ export const MENU_PANEL_TOP_PADDING = 8;
     '(keydown)': '_handleKeydown($event)',
     '(click)': '_handleClick($event)',
   },
-  exportAs: 'mdMenuTrigger'
+  exportAs: 'mdMenuTrigger, matMenuTrigger'
 })
 export class MdMenuTrigger implements AfterViewInit, OnDestroy {
   private _portal: TemplatePortal<any>;

src/lib/menu/menu.spec.ts

@@ -1113,7 +1113,7 @@ class OverlapMenu implements TestableMenu {
       <ng-content></ng-content>
     </ng-template>
   `,
-  exportAs: 'mdCustomMenu'
+  exportAs: 'mdCustomMenu, matCustomMenu'
 })
 class CustomMenuPanel implements MdMenuPanel {
   direction: Direction;