This adds a simple spacer widget first suggested by Scott Stoll (@ScottS2017) in the abandoned PR #15802

This PR replaces that one, cleans it up, and adds a test.

On smaller-screen devices such as the iPhone 4s, more scrolling is
required between the Material dialog demo and the Material pickers demo
than will happen within the default timeout.

... so the tree doesn't become dirty just by building it.

Fixes #16832

In FlutterDriver.scrollUntilVisible, pause momentarily after we kick off
the waitFor() call, in order to give that call a chance to complete
before we start scrolling if the element is on-screen.

This reverts commit ee019c0b.

This reverts commit 1095eafe.

When attempting to locate an element in the Flutter Gallery transition
performance driver test, attempt to scroll downward in increments of 25%
of the screen rather than by an absolute pixel amount.

This helps get the test passing on devices with relatively small screen
heights.

* Add a draggable axis restrictor and tests

1. Make CupertinoSwitch use BoxShadow.toPaint() so that it respects
   the `debugDisableShadows` flag.
2. Increase blue radius on debug banner
3. Only stroke "synthetic shadow" borders if elevation is positive.

In theory, this should aid with tree shaking and minutely optimize the code in release builds.

Before this change, if CDPATH was set, and you type (from the flutter root) "bin/flutter --version" you would get an error.

This is because the cd command in follow_links was printing the directory Bash chose from the CDPATH to stdout, causing the path to have a newline in it, which understandably confuses the rest of the script.

Take into account the subtitle height when adjusting the height of the widget. Added a test.

* Search for a suitable ARM sysroot instead of hardcoding it;
* Add facility to explain why NDK was not found;

This contains the following commits:

flutter/engine@9ae10ef Revert f6359e4111d37652409e2392fdb3cb72f1119c2a

This contains the following commits:

flutter/engine@b856303 Revert 37e20af6a324cd4cfa1324b8c644712ef8224359
flutter/engine@39c02c4 Fixed Windows build after Blink removal
flutter/engine@4a4cff9 Remove Blink code
flutter/engine@f2acd0a Roll src/third_party/skia/ f31fece65..1d686a962 (10 commits)

Adds unit tests for the following AOT build configurations:
* iOS armv7 profile
* iOS armv7 release
* Android armv7 profile (AOT blob)
* Android arm64 profile (AOT blob)
* Android armv7 release (AOT blob)
* Android arm64 release (AOT blob)

Further digging revealed that the reason --no-sim-use-hardfp was
required to be specified explicitly was that Android engine gen_snapshot
binaries are built on Windows with target_os=win.

This fixes #17169 by adding onChangeStart and onChangeEnd to the slider. These will be called when the user starts a change, and when they end a change, regardless of whether that change is a tap or a drag.

These differ from onChanged, in that they only report when the user starts and ends an interaction, not at every slight change.

The non-deprecated variants aren't widely available yet