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

Roll engine to 77b2d1d95a9438ff996d24ef3e0b3ec81822fa61 (contains the latest roll of Dart into teh engine). (#13269)

* Add a minimum distance that needs breaking on iOS each time scrolls stopped.

* Testing and tests

* tweak docs

* review

This is in preparation for adding a second card with a demo for animated
icons.

Table refactor is just a cleanup (that also fixes an alignment bug).