Something hidden about the Container

Although at the level of a Flash DisplayObjectContainer, all
children are equal, in a Flex Container some children are "more
equal than others". (George Orwell, "Animal Farm")

In particular, Flex distinguishes between content children and
non-content (or "chrome") children. Content children are the kind
that can be specified in MXML. If you put several controls
into a VBox, those are its content children. Non-content children
are the other ones that you get automatically, such as a
background/border, scrollbars, the titlebar of a Panel,
AccordionHeaders, etc.

Most application developers are uninterested in non-content children,
so Container overrides APIs such as numChildren and getChildAt()
to deal only with content children. For example, Container, keeps
its own _numChildren counter.

Container assumes that content children are contiguous, and that
non-content children come before or after the content children.
In order words, Container partitions DisplayObjectContainer's
index range into three parts:

A B C D E F G H I
0 1 2 3 4 5 6 7 8 <- index for all children
0 1 2 3 <- index for content children

The content partition contains the content children D E F G.
The pre-content partition contains the non-content children
A B C that always stay before the content children.
The post-content partition contains the non-content children
H I that always stay after the content children.

Container maintains two state variables, _firstChildIndex
and _numChildren, which keep track of the partitioning.
In this example, _firstChildIndex would be 3 and _numChildren
would be 4.