Bug 261506 [CommonNavigator] NavigatorDNDService seems like it can use only interfaces
The NavigatorDNDService class seems like it could use only interface APIs and function properly. Patch attached.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/dnd/NavigatorDnDService.java
} 
		return getAssistantsByTransferData(descriptors, aTransferType);
	}
	
	public CommonDropAdapterAssistant[] findCommonDropAdapterAssistants(
			Object aDropTarget, IStructuredSelection theDragSelection) {
 
		 
		CommonDropAdapterDescriptor[] descriptors = CommonDropDescriptorManager
				.getInstance().findCommonDropAdapterAssistants(aDropTarget,
						contentService);
		if (descriptors.length == 0) {
			return NO_ASSISTANTS;
		}
		return getAssistantsBySelection(descriptors, theDragSelection);  
	}
	private CommonDropAdapterAssistant[] getAssistantsByTransferData(
			CommonDropAdapterDescriptor[] descriptors,
			TransferData aTransferType) {
		Set assistants = new LinkedHashSet();
		for (int i = 0; i < descriptors.length; i++) {
			CommonDropAdapterAssistant asst = getAssistant(descriptors[i]);
			if (asst.isSupportedType(aTransferType)) {
				assistants.add(asst);
			}
		}
		return (CommonDropAdapterAssistant[]) assistants
				.toArray(new CommonDropAdapterAssistant[assistants.size()]);
	}
	private CommonDropAdapterAssistant[] getAssistantsBySelection(