Bug 305703 [Viewers][CommonNavigator] remembered contributions can leak when using filters
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
* contributing NCE. However in the case of different first class NCEs,
		 * the first one wins, so we don't update the contribution memory.
		 */
		synchronized (this) {
			if (contributionMemory.get(element) == null
					|| contributionMemoryFirstClass.get(element) == firstClassSource) {
				if (Policy.DEBUG_RESOLUTION)
					System.out
							.println("rememberContribution: " + Policy.getObjectString(element) + " source: " + source); 
				contributionMemory.put(element, source);
				contributionMemoryFirstClass.put(element, firstClassSource);
			}
		}
	}
	/**
	 * Forget about the specified element
	 * 
	 * @param element
	 *            The element to forget.
	 */
	public void forgetContribution(Object element) {
		synchronized (this) {
			contributionMemory.remove(element);
			contributionMemoryFirstClass.remove(element);
		}
	}
	/**
	 * @param element
	 * @return the remembered NavigatorContentDescriptor
	 */