* 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)

Adds unit tests for AndroidWorkflow.runLicenseManager().

AndroidSdk and AndroidStudio depends relatively heavily on filesystem
access to locate installed tools. Testing code that relies on either of
these classes benefits from a utility method to generate a
legitimate-looking Android SDK directory layout for testing.

flutter doctor --android-licenses requires Android sdkmanager version 26
or later. When sdkmanager is not available (SDKs earlier than v25),
direct users to SDK upgrade instructions. When it's installed but not
v26 or later, emit instructions to run sdkmanager --update.

Moves MockProcess, MockStdio and a few other useful mocks from
packages_test.dart to common/mocks.dart. These are useful for testing
code with interactive IO.

This adds a new constructor to MockProcess to provide additional flexibility.

Convenience getters for the the path to the Android SDK manager and the
currently installed version of the tool.

Pre-factoring to support better checks around the --android-licenses
command, which uses a feature of the SDK manager that is unsupported in
older versions of the tool.

Skia is trimming some of these transitive includes