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.
The collections package changed to add some stricter type annotations.
We now pass those type annoations. I also sent
https://github.com/dart-lang/collection/pull/26 to make the type
annotations match what they were previously.
* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)
Fixes#2348
The terms "left" and "right" have an LTR bias. Instead, we now use
"leading" and "trailing", for these list and grid decorations so that
we can later rearrange them to match the reading order.
Fixes#2540Fixes#2548
