Bug 126658 [CommonNavigator] [DND] Complete definition of DND API
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
*            The descriptor of the extension that contributed the set of
	 *            elements.
	 * @param element
	 *            An element from the given source.
	 */
	public synchronized void rememberContribution(
			NavigatorContentDescriptor source, Object element) {
		if (source != null && element != null)
			getContributionMemory().put(element, source);
	}
	/**
	 * 
	 * @param element
	 *            The element contributed by the descriptor to be returned
	 * @return The descriptor that contributed the element or null.
	 * @see #findContentExtensionsWithPossibleChild(Object)
	 */
	public NavigatorContentDescriptor getSourceOfContribution(Object element) {
		return (NavigatorContentDescriptor) getContributionMemory()
				.get(element);
	}
	/**
	 * @return Returns the contributionMemory.
	 */
	public Map getContributionMemory() {
		if (contributionMemory != null)
			return contributionMemory;
		synchronized (this) {
			if (contributionMemory == null)
				contributionMemory = new WeakHashMap();