The navigation bar's title.
This text will appear in the top static navigation bar when collapsed and below the navigation bar, in a larger font, when expanded.
A suitable DefaultTextStyle is provided around this widget as it is moved around, to change its font size.
If middle is null, then the largeTitle widget will be inserted into the tree in two places when transitioning from the collapsed state to the expanded state. It is therefore imperative that this subtree not contain any GlobalKeys, and that it not rely on maintaining state (for example, animations will not survive the transition from one location to the other, and may in fact be visible in two places at once during the transition).
final Widget largeTitle