code: function Selection(serviceRegistry, selectionServiceId) {
  		if (!selectionServiceId) {
  			selectionServiceId = "orion.page.selection"; //$NON-NLS-0$
  		}
  		
  		this._serviceRegistry = serviceRegistry;
  		EventTarget.attach(this);
  		this._serviceRegistration = serviceRegistry.registerService(selectionServiceId, this);
  		this._selections = null;  // so we can quickly recognize the "empty" case without comparing arrays.
  	}
docs: Constructs a new selection service. Clients should obtain a selection service
  by requesting the service <code>orion.page.selection</code> from the service registry.
  This service constructor is only intended to be used by page service registry
  initialization code.
  @name orion.selection.Selection
  @class Can provide one or more selections describing objects of interest.  Used to
  establish input and output relationships between components.  For example, the selection
  in one component can serve as the input of another component.