Bug 93360 [Viewers] getChild still used in indexForElement
indexForElement() in AbstractTreeViewer still uses getChild() and not getChildren(). This should probably also change to using getChildren() and do the same thing as insertionPosition().
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
* have been added to the model, in order to cause the viewer to accurately
     * reflect the model. This method only affects the viewer, not the model.
     * </p>
     * 
     * @param parentElement
     *           the parent element
     * @param childElements
     *           the child elements to add
     */
    public void add(Object parentElement, Object[] childElements) {
        Assert.isNotNull(parentElement);
        assertElementsNotNull(childElements);
        Widget widget = findItem(parentElement);
        
        if (widget == null)
            return;
        internalAdd(widget, parentElement, childElements);
    }
	/**
     * Adds the given child elements to this viewer as children of the given
     * parent element.
     * <p>
     * EXPERIMENTAL.  Not to be used except by JDT.
     * This method was added to support JDT's explorations
     * into grouping by working sets, which requires viewers to support multiple 
     * equal elements.  See bug 76482 for more details.  This support will
     * likely be removed in Eclipse 3.2 in favour of proper support for
     * multiple equal elements. 
     * </p>
     *