The 'routes' table is a point of confusion with new developers. By
providing a 'home' argument that sets the '/' route, we can delay the
point at which we teach developers about 'routes' until the point where
they want to have a second route.

* find all repo packages

* .dartignore

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

Initial version of complex layout performance test