* Try to make transitioning channels actually work.

* Update upgrade.dart

Includes:
* Roll Dart SDK (flutter/engine#4613)

This script will update release metadata in the cloud, and copy the already-built package to the right location and name on cloud storage.

The release metadata will be located in gs://flutter_infra/releases/releases.json, and the published packages will end up in gs://flutter_infra/releases/<channel>/<platform>/flutter_<platform>_<version><archive suffix>, where <channel>, <platform>, <version>, and <archive suffix> are determined by the script.

At the moment, it only supports dev rolls, but (once we know how those will work) should easily support beta rolls as well.

* Add a callback that fires when a Draggable leaves a DragTarget.  This enables the DragTarget to manage its state from entry to exit.

* It helps to have a null check here

* Add test for onLeave callback and add verbiage to onWillAccept explaining the callback lifecycle of a DragTarget.

* adds crossAxisEndOffset and rollbackDuration properties to dismissible widget

* re-added removed license comment

* corrected license comments

* Adds test for rollbackDuration and crossAxisEndOffset

* added tests and comments

* modified tests for dismissible widgets

Includes:
* Remove unnecessary trim (#4616)

It adds lines like `SHARD=analyze` to each shard, and a message like `Contents of packages/flutter_localizations/lib/src/l10n/localizations.dart matches output of gen_localizations.dart script.`  to the analysis shard.

Includes:
* Extract common iOS asset path lookup code: #4594
* Introduce VulkanProvider fb9782a5294e700e45e372da0cd25165697b2e45
* Transition render target images before submit 0de8218778f1bb5d913db8b8a67789a44f8bb28f
* Update buildtools to Fuchsia HEAD #4599

* Add onCancelled callback to PopupMenu

* Fix spelling, don't call onCanceled if disposed, improve documentation.

* Make the current command injected into the AppContext, allowing
  other classes to inject the current command.
* Introduce `AssetBundleFactory`, an injected factory class for
  spawning instances of `AssetBundle`. This allows other run contexts
  to use custom asset bundling logic.
* Clean up RunCommand by removing a 'packages' argument that duplicated
  a global argument by the same name (and for the same purpose).
  Duplicate arguments are confusing and error-prone.

This was overlooked in 5e18c076

* Fix shadows in NestedScrollView

The previous logic (and especially the previous test) wasn't quite right.

* Update nested_scroll_view.dart

This widget evolved over time by multiple people, I'm just trying to explain
in a more direct way what the widget is currently doing.

When running with --preview-dart-2, host variant of the engine is
required. Simulator builds, like device builds, should resolve
host_RUNTIMEVARIANT_OPTVARIANT, not host_RUNTIMEVARIANT_sim_OPTVARIANT.

Fixes https://github.com/flutter/flutter/issues/14232

(I haven't tested this with older versions of git, I'm just going on
what @jason-simmons wrote in the bug.)

* Point to master API docs

I _think_ this is the right place, since people contributing and working on master might care about where the API docs are for master branch.

* Update CONTRIBUTING.md

update from review comment

* Roll engine to 6921873c71e700235c0f68f0359be2332f93c8bc

Also some trivial improvements to style.

Roll engine to 7c34dfafc9acece1a9438f206bfbb0a9bedba3bf

* Unpin package:test and upgrade packages

* Update packages/flutter/test/foundation/stack_trace_test.dart

* Also add packages/flutter_tools/test/data/asset_test/font/.dartignore to ensure that update-packages --force-upgrade does not crash.

The code was attempting to delete temporaryDirectory while it should have been deleting tempBundleDirectory

* controller, position and test

* Make controllers swappable

* WIP

* Create a ListWheelScrollPhysics

* Created picker and gallery demo and testing now

* Works. Ready to document and test.

* Document and add tests. Make the scroll controller more generic.

* minor cleanup

* review

* review

* fix tests

* stop using TransformLayers for now

This reduces the jank of bringing up the license screen further.
The remaining lost frame or two are caused by Dart itself, see:

   https://github.com/dart-lang/sdk/issues/31954
   https://github.com/dart-lang/sdk/issues/31959
   https://github.com/dart-lang/sdk/issues/31960

Fixes https://github.com/flutter/flutter/issues/5187

Framework side for https://github.com/flutter/engine/pull/4584 & https://github.com/flutter/engine/pull/4587.

Also rolls engine to 4c82c566edf394a5cfc237a266aea5bd37a6c172.

(this will be generated by flutter update-packages)