Bug 126658 [CommonNavigator] [DND] Complete definition of DND API
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
*         <i>visible</i> and <i>active</i> for this content service and
	 *         have a <b>possibleChildren</b> expression that is <i>enabled</i>
	 *         for the given element.
	 */
	public Set findDescriptorsWithPossibleChild(Object anElement) {
		NavigatorContentDescriptor descriptor = getSourceOfContribution(anElement);
		Set result = new HashSet();
		if (descriptor != null)
			result.add(descriptor);
		result.addAll(CONTENT_DESCRIPTOR_REGISTRY
				.findDescriptorsForPossibleChild(anElement, assistant));
		return result;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.ui.internal.navigator.INavigatorContentService#onExtensionActivation(java.lang.String,
	 *      java.lang.String, boolean)
	 */
	public void onExtensionActivation(String aViewerId,
			String[] aNavigatorExtensionId, boolean toEnable) {
		update();
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.ui.internal.navigator.INavigatorContentService#update()
	 */
	public void update() {
		rootContentProviders = null;