Fixes #8413

* Adds un/selected text styles to the TabBar widget

* Adding documentation on fallback values. Modified fallback on unselectedLabelStyle.

iOS builds rely on package dependencies and associated build artifacts
(pubspec.lock, .packages files) to be up-to-date.

It's common to just want a simple colored box. Simple thing should be simple,
so this patch adds a convenience argument to Continer for creating a box
decoration that is just a color.

Fixes #5555

In some situations, dialogs don't want the barrier to be dismissable.

Fixes #8354

With slivers, the app bar is part of the scroll view. Naively, the overscroll
appears at the beginning of the scroll view, but that's not the desired
behavior. Instead, the app bar should remained pinned to the top of the scroll
view and the overscroll should appear below the app bar, which is what this
patch does.

Fixes #8228

The gradle build scripts can be configured to output different
application IDs for different build types, so we need to examine the
built .apk to figure out the name of the package and activity.

Fixes #8327

It's not just $HOME/.AndroidStudio2.2, it might also be
.AndroidStudioPreview2.3, or .AndroidStudioFooBar1.7, or whatever.

Made the Version parser less throw-happy, and relaxed the directory name
checks to allow for the above.

Fixes #8353.

Fixes #8380

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