(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.

5f445d7c0 Roll Dart SDK from 812bf07cf737 to 40695d910591 (1 revision) (flutter/engine#27306) (#86239)

0a892d9ec Roll Dart SDK from ab8bac9fd542 to 812bf07cf737 (1 revision) (flutter/engine#27300) (#86215)

36271dc62 Roll Dart SDK from 83a559e0a9bd to ab8bac9fd542 (1 revision) (flutter/engine#27298) (#86212)