Previously, the client of PopupMenuButton needed to build all the menu times
when building the PopupMenuButton. This can get expensive if, for example, each
item in a scrollable list has a popup menu associated with it.
Now the client passes a builder function to the PopupMenuButton that gets
invoked only when its time to show the menu items.
LazyBlock is intended as a replacement for MixedViewport. Rather than
maintaining a table of all the observed child sizes (like
MixedViewport), LazyBlock works by dead reckoning the location of the
children based on the existing viewport. This approach makes it easier
to resize children because LazyBlock doesn't cache any additional
information that would need to be invalidated.
This patch contains a first draft of LazyBlock that works in a simple
usage scenario. Subsequent patches will replace
ScrollableMixedWidgetList with LazyBlock and port the existing
ScrollableMixedWidgetList tests over to LazyBlock.
Related to #3075
