Bug 250133 [Contexts] ContextAction's leak when hiding actions sets
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/contexts/ContextManager.java
final IContextManagerListener listener = (IContextManagerListener) listeners[i];
			listener.contextManagerChanged(event);
		}
	}
	/**
	 * Returns the set of active context identifiers.
	 * 
	 * @return The set of active context identifiers; this value may be
	 *         <code>null</code> if no active contexts have been set yet. If
	 *         the set is not <code>null</code>, then it contains only
	 *         instances of <code>String</code>.
	 */
	public final Set getActiveContextIds() {
		return Collections.unmodifiableSet(activeContextIds);
	}
	/**
	 * Gets the context with the given identifier. If no such context currently
	 * exists, then the context will be created (but be undefined).
	 * 
	 * @param contextId
	 *            The identifier to find; must not be <code>null</code>.
	 * @return The context with the given identifier; this value will never be
	 *         <code>null</code>, but it might be undefined.
	 * @see Context
	 */
	public final Context getContext(final String contextId) {
		checkId(contextId);
		Context context = (Context) handleObjectsById.get(contextId);
		if (context == null) {
			context = new Context(contextId);