The JDK doesn't put `javac` in the PATH on Windows :(

* Bump to test `0.12.20`.

Some test `0.12.20` highlights:
  * introduces `expectLater()`  that returns a `Future` that completes when the matcher has finished running
  * deprecates the `verbose` parameter to `expect()` and the `formatFailure()` (to be removed in `0.13.0`)

Otherwise:
  * to keep up w/ the deprecation of `verbose`, removes `widget_tester` API to pass `verbose` flag (alternatively we could suppress the warning for now)

* Update stack manipulation.

* Fix framecount.

Mainly: Don't call out to PoerShell (expansive!) if Dart SDK is upto date.

If a target file is specified on the flutter tools command line, pass it
through to Gradle.

It is still possible to statically specify a target file in the flutter
section of build.gradle, but it is now possible to specify it on the
command line as well. The command line option takes precedence.

Fixes #8175.

Fixes #6761.

Trigger a vibration on hour/minute changes, with an upper bound on
number of feedback events per second.

Note: state changes are expected to trigger haptic feedback on Android,
but not on iOS time pickers.

State changes are expected to trigger haptic feedback on Android, but
not on iOS time pickers.

Makes RenderViewport.layoutOneSide more readable.

Rather than generating a cryptic assert, we now generate a more useful error
message.

Fixes #8296

Line was accidentally removed by #8320

Also, change the version check from `java` to `javac` since we need the JDK and not just the JRE.

In the Android native material DatePicker, haptic feedback is expected
on date change. This is not expected behaviour for iOS date/time
pickers.

Extend app bar left to edge, right to 4dp
Make leading button square and 56dp
Keep title at 72dp on Android according to Material
Renamed IconButton.size to .iconSize
IconButton minimum size expands to 48dp (#8264)
IconButton default constraints to 48. Can still stretch to infinity but can't be smaller than 48.
Ink splash for IconButton 40% bigger than the touch target to match Material
Tests

appveyor.com runs our tests on Windows for each PR.

Only implemented for Android devices for now. Compare the installed SHA1
to the latest build. If they match, there's no reason to reinstall the
build.

Fixes #8295

* Fix bug parsing Gradle version.

Version from pub_semver requires versions of the format X.Y.Z. Gradle
doesn't follow semantic versioning, though, so version parsing would
fail on versions like '3.2'. Fixed by writing a custom Version class.

Also removed a check for apksigner when building Gradle-based projects.

Fixes #8298

Previously, a ListView would always accept user input, even if it wasn't
actually scrollable. Now, by default, we don't accept user input if there's no
scroll range. You can override this behavior using the ScrollPhysics.

Fixes #8276
Fixes #8278
Fixes #8271

This reverts commit 70d7fe3a.

The Java version missmatch on the bots has been resolved.

Otherwise tapping the menu button causes the bottom sheet to disappear and
reappear.

Fixes #8275

This enables us to run more tests on Windows.

This functionality is independently useful.

Fixes #8293

Discuss how to change paint order.

It doesn't need to be in the same file as RenderSliver.

Do not attempt to deploy/debug wifi connected iOS devices. ios-deploy is
able to install over wifi, but we've had several bugs reporting failure
to run/debug once installation has completed when the device is also
connected via USB. Note that we don't currently support deploy/debug
over wifi since libimobiledevice (which is also required) requires a USB
connection.

This functionality got lots somehow in the migration to maybePop.

Fixes #8258