Returns the last index of the element within the collection, or -1 if not found.

Note: A collection will not contain any duplicate items. All items within a collection are unique.

var shapeA = Pablo.circle(),
    shapeB = Pablo.rect(),
    shapeC = Pablo.ellipse(),
    shapes = Pablo([shapeA, shapeB, shapeC]);

alert(shapes.lastIndexOf(shapeB)); // 1