This reverts commit e2490f29.

(Attempt to reland #85993)

This reverts commit 53d8cba3.

The first line of the documentation of equality functions listEquals, mapEquals, and setEquals claimed that they are deep equality functions. A later paragraph explained that they check for deep equality only if the elements, which are collections, implement the equality operator to do so. That is almost never the case.

The first line of the documentation was changed to element-by-element equality, and a reference was added to the DeepCollectionEquality class.

This reverts commit 2175e64e.

This reverts commit 9d3563a8.

This reverts commit d4afa289.

Migrate core devicelab framework to null safety.