Right now, `[md-menu-trigger-for]` requires that its value be an instance of `MdMenu`: ``` html <button md-icon-button [md-menu-trigger-for]="menu"> Open Menu </button> <md-menu #menu="mdMenu"> <button md-menu-item> Hello! </button> </md-menu> ``` It would be more flexible to allow the value to be anything that conforms to the expected interface (something like `interface MdCustomMenu`).