Bug 305703 [Viewers][CommonNavigator] remembered contributions can leak when using filters
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
private boolean labelProviderInitialized;
	private boolean isDisposed;
	
	/**
	 * @param aViewerId
	 *            The viewer id for this content service; normally from the
	 *            <b>org.eclipse.ui.views</b> extension.
	 */
	public NavigatorContentService(String aViewerId) {
		super();
		aViewerId = aViewerId != null ? aViewerId : ""; 
		viewerDescriptor = VIEWER_DESCRIPTOR_REGISTRY
				.getNavigatorViewerDescriptor(aViewerId);
		assistant = new VisibilityAssistant(viewerDescriptor,
				getActivationService());
		getActivationService().addExtensionActivationListener(this);
		contributionMemory = new HashMap();
		contributionMemoryFirstClass = new HashMap();
	}
	/**
	 * @param aViewerId
	 *            The viewer id for this content service; normally from the
	 *            <b>org.eclipse.ui.views</b> extension.
	 * @param aViewer
	 *            The viewer that this content service will be associated with.
	 */
	public NavigatorContentService(String aViewerId, StructuredViewer aViewer) {
		this(aViewerId);
		structuredViewerManager = new StructuredViewerManager(aViewer, this);
	}