* Bump Dart SDK to 1.25.0-dev.9.0

* add link to sdk bug

Mainly, this adds documentation to members that were previously
lacking documentation.

It also adds a big block of documentation about improving performance
of widgets.

This also removes some references to package:collection and adds
global setEquals and listEquals methods in foundation that we can use.
(setEquals in particular should be much faster than the
package:collection equivalent, though both should be faster as they
avoid allocating new objects.) All remaining references now qualify
the import so we know what our remaining dependencies are.

Also lots of code reordering in Flutter driver to make the code
consistent and apply the style guide more thoroughly.

* move more tests to Linux hosts

* fix test

Aggressively apply the const lint.

* devicelab benchmarks run on Windows

* fix analyzer issue

* fix test

* fix pubspec

... and run some more tests on Windows.

* disables all `flutter test` and `flutter drive` tests on Windows as those two commands are not fully implemented on Windows yet
* fixes other failures on Windows

This reverts commit 8f273e49.

I ran the test 10 times on Linux and Mac and it succeeded
consistently. Let's reenable until there are new reports that it's
flaky.

Fixes #5901

* fix flutter watch benchmark

Syncs https://github.com/flutter/cocoon/commit/d2d7950ecd2c72421d75d9b5d54805cc554e92a8

* Split Android/iOS impl behind a unified interface

Syncs https://github.com/flutter/cocoon/commit/db87e10fa54317115479d0b85280c7e5eed08ff7

* Switch from pub get to flutter packages get

Syncs https://github.com/flutter/cocoon/commit/b378005cbbc5330058d5240a2970ca477c2c9722

* "silent" option in test runner; fix analysis errors;