* Video demo is unsupported on simulator

Display a message stating this.

* Address review

* Add spinner to loading video

Summary:

- Add `key` field to `SemanticsNode`, while moving key into `foundation` library so it can be used by the render layer.
- Introduce `SemanticsProperties` and move many of the `Semantics` fields into it.
- Introduce `CustomPaintSemantics` - a `SemanticsNode` prototype created by `CustomPainter`.
- Introduce `semanticsBuilder` and `shouldRebuildSemantics` in `CustomerPainter`

**Breaking change**

The default `Semantics` constructor becomes non-const (due to https://github.com/dart-lang/sdk/issues/20962). However, a new `const Semantics.fromProperties` is added that still allowed creating constant `Semantics` widgets ([mailing list announcement](https://groups.google.com/forum/#!topic/flutter-dev/KQXBl2_1sws)).

Fixes https://github.com/flutter/flutter/issues/11791
Fixes https://github.com/flutter/flutter/issues/1666

Separate focus management by build owner

Currently the focus manager is held by the singleton WidgetBinding.
This change places a focus manager in each build owner object,
which allows clients to run offscreen pipelines without disturbing
focus management for the main UI.

Somehow the test ended up checking for notification messages at the wrong depth.

Also, extra docs.

Fixes https://github.com/flutter/flutter/issues/13279
Fixes https://github.com/flutter/flutter/issues/2115

...and document what I did.

Revert "Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)" (#13317)

This reverts commit 5c4ffa13.

* Provide more information in PUB_ENVIRONMENT for `pub get/upgrade`

Should help diagnose and fix https://github.com/dart-lang/pub-dartlang-dart/issues/636

Dropped optional semicolons.

A more future-proof approach to accessing internal package name lint rule description details.

See: https://github.com/dart-lang/linter/issues/654

Updates quiver from 0.26.0 to 0.26.2 to pick up strong-mode fixes.

Rather than depending on android.googlesource.com for hosting the gradle
wrapper, host our own copy on storage.googleapis.com (along with our
other artifacts).

Also added a script to repackage AOSP's version into ours, in case we
ever need to update it.

Addresses the gradle wrapper part of #11681.

Extracts Tab2ConversationRow class that consolidates widget tree for a
chat message in the Cupertino navagation chat demo. This refactoring
simplifies adding SafeArea support to handle iOS 11 safe areas in a
followup patch.

1. Ensure that this value is defaulted to the value associated with the
   context rather than re-defaulted to false.
2. Add this value to operator==, hashCode, toString methods.

This will be followed by a patch that supports independently removing
padding and view insets.

* Fix --preview-dart2 option handling for ios profile/release.

* Remove unused parameter

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

* Add MediaQuery support for view insets

Also updates EdgeInsets documentation to reflect WindowPadding's use for
both padding and view insets.

See engine commits:
  flutter/engine#4403
  flutter/engine#4406

Fixes the remaining known issues with widgets supporting RTL.

* ensures that only semantics boundaries will be added to owner._nodesNeedingSemantics as expected by compiler.
* no longer throws assert if markNeedsSemanticsUpdate is called on non-semantic-boundary render object with a non-semantic-boundary parent.
* Fixes #13109.
* removes onlyLocalUpdates from markNeedsSemanticsUpdate as its no longer needed.

* Change some of the dev/devicelab tests to support testing against local engine.

We can already configure flutter tools to use local engine by setting
FLUTTER_ENGINE environment variable. However when this variable is set
this also requires setting --local-engine to specify which flavor of
engine to use.

This change changes tests in dev/devicelab to pass a sensible default for
--local-engine, e.g. when testing hot reload on Android we pass android_debug
and when testing release AOT build for IOS we pass ios_release.

* Fix analysis issues

* Update utils.dart