Bug 305703 [Viewers][CommonNavigator] remembered contributions can leak when using filters
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
Set result = new TreeSet(ExtensionSequenceNumberComparator.INSTANCE);
		result.addAll(CONTENT_DESCRIPTOR_REGISTRY
				.findDescriptorsForPossibleChild(anElement, assistant,
						toComputeOverrides));
		return result;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @seeorg.eclipse.ui.internal.navigator.INavigatorContentService#
	 * onExtensionActivation(java.lang.String, java.lang.String, boolean)
	 */
	public void onExtensionActivation(String aViewerId,
			String[] aNavigatorExtensionId, boolean toEnable) {
		synchronized (this) {
			SafeRunner.run(new NavigatorSafeRunnable() {
				public void run() throws Exception {
					NavigatorContentDescriptor key;
					NavigatorContentExtension extension;
					for (Iterator iter = contentExtensions.keySet().iterator(); iter
							.hasNext();) {
						key = (NavigatorContentDescriptor) iter.next();
						INavigatorActivationService activation = getActivationService();
						if (!activation.isNavigatorExtensionActive(key.getId())) {
							extension = (NavigatorContentExtension) contentExtensions
									.get(key);
							iter.remove();
							extension.dispose();
						}
					}