goog.ui.ToolbarMenuButton is not a function ; Google Closure

Question

Hey I am trying to add a toolbarMenuButton to my toolbar but for some reason I get this error goog.ui.ToolbarMenuButton is not a function ; The toolbar is going to be a mix of toolbarButton and toolbarMenuButton. Right now its just toolbarButton.

this.myToolbar = new goog.ui.Toolbar();
this.addChild(this.myToolbar);

var SelectBtn2 = new goog.ui.ToolbarMenuButton(dom structure);
this.myToolbar.addChild(SelectBtn2, true);

var SelectBtn1 = new goog.ui.ToolbarButton(dom structure);
this.myToolbar.addChild(SelectBtn1, true);

Originally I have selectBtn1 as ToolbarButton and it works fine. But the moment I convert it into toolbarMenuButton (seen in selectBtn2) it gives the above error. Why ? According to the documentation both of them take same parameter.