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/navigator/CommonDropAdapter.java
*         {@link LocalSelectionTransfer#getTransfer()},
	 *         {@link FileTransfer#getInstance()},
	 *         {@link PluginTransfer#getInstance()}.
	 * @see LocalSelectionTransfer
	 * @see FileTransfer
	 * @see PluginTransfer
	 */
	public Transfer[] getSupportedDropTransfers() {
		return SUPPORTED_DROP_TRANSFERS;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.viewers.ViewerDropAdapter#dragEnter(org.eclipse.swt.dnd.DropTargetEvent)
	 */
	public void dragEnter(DropTargetEvent event) {
		super.dragEnter(event);
		if (event.detail == DND.DROP_NONE)
			return;
		
		for (int i = 0; i < event.dataTypes.length; i++) {
			if (LocalSelectionTransfer.getTransfer().isSupportedType(
					event.dataTypes[i])) {
				event.currentDataType = event.dataTypes[i]; 
				return;
			}
		}
		for (int i = 0; i < event.dataTypes.length; i++) {
			if (FileTransfer.getInstance().isSupportedType(event.dataTypes[i])) {
				event.currentDataType = event.dataTypes[i];