This is a follow up on issue #17169 and the pull request #17298

This pull request adds the onChangeStart and onChangeEnd callbacks for CupertinoSlider. These are called when a user starts and ends a change respectively.

Pushing for @dcaraujo0872, the PR author.

This allows test environments other than `flutter test` to have a hook
into the test exception reporting. Some test environments, for example,
don't just dump error details to the console, but rather require them
to be reported to a separate server.

There's no reason for them to read/write.

Adds fuchsia_remote_debug_protocol gn file.

This contains the following commits:

flutter/engine@1179c38 Clear the raster cache result when bypassing raster cache.
flutter/engine@ea9f96b Roll src/third_party/skia/ dd6e62e28..44890ef00 (1 commit)
flutter/engine@5c4201f Roll src/third_party/skia/ 51371a433..dd6e62e28 (4 commits; 1 trivial rolls)
flutter/engine@a56f2f2 Roll src/third_party/skia/ 000e00df1..51371a433 (11 commits; 1 trivial rolls)

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

This replaces abandoned PR #14824 by @ekbiker, and gives it some love.

This is follow-up to https://github.com/flutter/flutter/pull/17682 to fix dart1 tests.

* Roll engine to 40a37126078f3eee2cd7584da04704908f47cd68

Changes since last roll:
```
40a371260 Remove the content handler for its move to topaz. (#5289)
a262da46f Roll src/third_party/skia/ c06754b04..000e00df1 (12 commits; 1 trivial rolls) (#5288)
f7eab94f4 Roll src/third_party/skia/ bd74e6a02..c06754b04 (1 commit) (#5287)
597fa9ed4 Delegate AppDelegate life-cycle callbacks to plugins via separate object (#5173)
ef0915c10 update docs for Path.getBounds (#5261)
bb1736b7b Roll src/third_party/skia/ 15f1761d9..bd74e6a02 (1 commit; 1 trivial rolls) (#5286)
8c9558b1f Roll src/third_party/skia/ ed4921b9d..15f1761d9 (1 commit) (#5285)
5cbfa9593 Roll src/third_party/skia/ 05da1c16d..ed4921b9d (1 commit) (#5284)
919e8c211 Support for TwoPointConical gradients (#5275)
cdd7e0887 Roll src/third_party/skia/ 94f585ed0..05da1c16d (9 commits; 1 trivial rolls) (#5283)
```

* Include more changes(reverts and a fix)

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.

* Fix consolidate response and add test

* Update AUTHORS

* ignore content-length

* fix formatting/update comment

* more commas

* Fix consolidate response and add test

* Update AUTHORS

* ignore content-length

* fix formatting/update comment

* more commas

* remove extra newline

restored https://github.com/flutter/flutter/pull/17496
restored https://github.com/flutter/flutter/pull/17580

Remaining issue was fixed in 580c844c.

See https://github.com/flutter/flutter/issues/11961

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.