Fixes a small rendering problem when dragging the slider, as well as a couple of small bugs (de-duping of value changes, and actually using a parameter passed to _unlerp).

Includes:
* Add nil check for country code and language code (flutter/engine#4732)

This implements an update to the look of the Slider widget.

Specifically, it does the following:

* Adds the ability to customize the colors of all components of the slider
* Adds the ability to customize the shape of the slider thumb and value indicator
* Adds the ability to show the value indicator on continuous sliders
* Updates the default value indicator to be a "paddle" shape with new animations.
* Changes the tick marks to be visible all the time on discrete sliders
* Fixes a memory leak of an animation controller.
* Removes "thumbOpenAtMin" flag, which is no longer needed, and can be emulated by the
custom thumb shape support. It was not widely used.
* Adds tests for all of the new features.

Fixes https://github.com/flutter/flutter/issues/14950

In the case of "dev", `git checkout dev` could be miconstrued
as checking out the dev _directory_ rather than the branch...

https://github.com/flutter/flutter/issues/14865

* Show 32/64bit in VS Code headings on Windows

Fixes #14949.

* Run flutter tests in preview-dart-2 mode on travis flutter builds.

* Run dart2 tests on osx. Run dart tests in dart2.

* Fix name camelCase

* Default options to empty array, rather than null

* Troubleshoot failures

* More logging

* Troubleshoot: run single test

* Troubleshoot: run 15 tests

* Troubleshoot: run 15 tests with fix

* Try limit concurrency to 1

* Limit concurrency for preview-dart-2 tests to 4

* Move dart2 tests to allow_failures section

* Reinstate tests_dart_2 shard

* Raise concurrency to 8

* Reuse compiler across multiple test runs

* Allow to switch entry points when recompiling.

Actually use single compiler to incrementally recompile all tests executed by 'flutter test'.

* Remove leftover commented code

* Fix comment

* Lints

Changes since the last roll:
  - Specify sdkSummary when compiling via frontend server (#4733).
  - Fix traversal order for a11y scrolling (#4726)

accidentally missed due to named parameters being omitted.

Adds more actionable error messages for users of the most common Linux
distributions. We could improve this by checking the current
distribution and emitting only the appropriate error message. We could
further improve that by only emitting the install instructions if we
determine the appropriate package is not installed.

See: https://github.com/flutter/flutter/issues/6207

I found that some ValueListeners want to know when they should start
doing work (e.g. if the value comes from polling a network resource).

This roll contains:
* Make deprecated API unavailable (https://github.com/flutter/engine/pull/4722)
* Remove unavailable API in FlutterDartProject.h (https://github.com/flutter/engine/pull/4723)
* remove unavailabe API from FlutterDartProject.mm (https://github.com/flutter/engine/pull/4724)
* Prepare to enable global type flow analysis in AOT (https://github.com/flutter/engine/pull/4725)

* Add more haptic feedback varieties

* tests

* un-meta-ify the test

Fixes https://github.com/flutter/flutter/issues/15002

This is to fix #14931.

* If example code is not found, render a message instead of an endless spinner
* Trim codeTags to avoid lookup issues with windows carriage returns
* Trim \r on Windows off the end of code blocks so they're consistently \n
* Add a test to ensure sample code parses if built on Windows
* Add a comment about why we trimRight()

Fixes #14820.

* Revert "Revert "Add android license verification to doctor and some refactoring" (#14727)"

This reverts commit d2602947.

* Add tests, fix sdkManagerEnv and use it consistently, and rearrange Status object model

* AnsiSpinner needs to leave the cursor where it found it.

* fix tests

* Const constructor warning only shows up on windows...?

* Avoid crash if we can't find the home directory

* Make pathVarSeparator return a string in the mock

* Implement review comments

* Fix out-of-order problem on stop

Fixes https://github.com/flutter/flutter/issues/14975

* roll engine version in flutter/flutter to b05efc204090f430387398d3c35589571a2fe16b

* Fix analyzer warnings.

The downcast was crashing when toggling hasNotch for BottomAppBar.

Dart is migrating to .dart_tool/ as the location for cached artifacts
and other temporary files generated by tooling in the SDK. As part of
this, pub will be migrating from .pub to .dart_tool/pub.

In future, this path will also be used for other tooling, such as package:build.

We can't use .tar.xz on Mac because although it can unpack them on the command line (with tar), the "Archive Utility" that runs when you double-click on them just does some crazy behavior (it
converts it to a compressed cpio archive, and when you double-click on that, it converts it back to .tar.xz, without ever unpacking it!)

So, this changes the script to use .zip for Mac, and the files are about 220MB larger than they need to be. 

* Revert "Disable a text field test that fails on some Macs with libtxt (#14895)"

This reverts commit aa04a056.

* Revert "Roll engine to 33b88173f3820690169348859bbdc29133179e0b (#14832)"

This reverts commit 44592238.

* Revert non-engine reverts

https://github.com/flutter/flutter/issues/14893

* Doc improvements for animations

libtxt is now the default text renderer