The purpose of this PR is to make it so that when the user runs 'flutter', if they have a .pub-cache directory in their flutter root, we use that instead of the default location for the pub cache. Otherwise, it should act as before.

The eventual goal is to support a pre-populated flutter .zip/.tar.gz file that has everything the developer needs in one bundle. In order for that to actually work, we need to have the pub cache be self-contained, and not in the user's home dir.

Another advantage of this is that if you have multiple flutter repos that you're switching between, then the versions in the pub cache will remain static when you switch between them.

This is an attempt to re-land: #13248. Includes a fix for the test that makes it work on bots in the presence of PUB_CACHE being set, and no other changes.

* Revert "Add tests."

This reverts commit 31bad961ff2220a2708917ff8f748fed7efa9b51.

* Revert "Use .pub-cache from Flutter root, if it exists. (#13248)"

This reverts commit 72d6bcc3.

It turns out that when a PR build clones the flutter repo, it puts it into a directory called "flutter", and when the "real" build clones the flutter repo, it puts it into a directory called "build". This allowed the tests to succeed in my PR, but fail when I committed it.

This fixes the test so it doesn't depend on the cloned directory name.

The purpose of this PR is to make it so that when the user runs 'flutter', if they have a .pub-cache directory in their flutter root, we use that instead of the default location for the pub cache. Otherwise, it should act as before.

The eventual goal is to support a pre-populated flutter .zip/.tar.gz file that has everything the developer needs in one bundle. In order for that to actually work, we need to have the pub cache be self-contained, and not in the user's home dir.

Another advantage of this is that if you have multiple flutter repos that you're switching between, then the versions in the pub cache will remain static when you switch between them.

* Make drawer demo header not scroll

* Add test for gallery drawer demo and fix user accounts drawer header overflow

* 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