* Randomize tests, exclude tests that fail with randomization.

* Disable some more tool tests

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

This reverts commit b5f9612c because it is taking about 40% longer to run the tests, which is causing timeouts.

This turns on order shuffling for all tests that don't fail with it on, marking those tests that do fail with a tag so that they will be run without shuffling on.

To determine which tests fail with it on, I ran all the tests 100 times with different random shuffle seeds, and then also ran it with the date seeds from today until the end of July, and tagged all of the test suites (files) that fail, with a seed that caused them to fail.

Resolved contradictory documentation of showGeneralDialog and default value of 'barrierLabel' argument (#78890)

* Add dismiss action to modal barrier

* fix test

* fix another test

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

Flutter's `moreOrLessEquals` has a few advantages over `closeTo` from
the `matcher` package:

   * It emits the epsilon value in the test result on failure.
   * It uses a named parameter for epsilon, which improves readability
     at the call site.
   * It has a reasonable default for epsilon in cases where something
     more specific isn't required.

Using it also has the nice property that it aids in its own discovery
when when people go looking for such functionality in new tests.

This change also includes a couple unrelated whitespace formatting cleanups.

`showDialog()` and `showGeneralDialog()` were allowing transparent
colors but then triggering an assertion down the stack while building
the modal barrier. The assertion existed to keep from animating from
transparent to transparent, but there's no need for the assertion,
since we can just treat the transparent case as the same as the null
case -- no animation necessary.

The function passes most of its arguments through to
`_DialogRoute`, but it wasn't providing the default
values that `_DialogRoute` was, causing it to override
the defaults will null values.

* Add material page, cupertino page, and transition page classes

* update

* update comments

* fix test

* addressing comments

* make page getter private

* Add semantics sort key for modal scope and modal barrier

* fix test

* fix test

* fix test

* fix space

* fix more tests

* addressing comments

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag