* Pin all dependencies ONCE AND FOR ALL

This replaces the secret `flutter update-packages --upgrade` with a destructive `flutter update-packages --force-upgrade` that actually goes and pins every dependency and transitive dependency in every flutter package to the same version.

* Add comments.

* requires bumping `process` to a version that has
  relaxed version restrictions on `intl`

https://github.com/flutter/flutter/issues/10650

The new assets have been optipng'ed.

This reverts commit 4bbf158b.

* Revert "Update reference to url_launcher plugin in Gallery pubspec (#9879)"

This reverts commit 96a04dea.

* Revert "Revert "Revert plugin use in gallery (#9838)" (#9877)"

This reverts commit ede72a80.

This reverts commit 96a04dea.

This reverts commit 4bbf158b.

* Revert "add Podfile (#9835)"

This reverts commit 74e65675.

* Revert "update ref to url_launcher plugin in Flutter Gallery (#9833)"

This reverts commit 30c49781.

* Revert "Use url_launcher plugin in Flutter Gallery (#9751)"

This reverts commit 02dfd6be.

The gallery doesn't actually use `package:flutter_markdown`.

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

Switch our pubspec.yamls to using SDK sources so that we can have consistent
source types when we depend on these packages from external packages using SDK
sources.

This code is now in its own standalone library. The library is in a private git
repository in the flutter organization because the code is unmaintained. If
you're interested in using and maintaining this code, please contact
flutter-dev@googlegroups.com for more information.

* Udpate to Dart `1.20.0-dev.1.0`.

Udpate to Dart `1.20.0-dev.1.0` and corresponding `analyzer` package.

* Added `--no-packages-dir`.

* Revert extension to master.

* Added ignores.

* Review nits.

* More nits.

This repo contains the final licensed images.

Also update the assets version to pull in better quality logo images.

BUG=https://github.com/flutter/flutter/issues/4407

Now that there's a new string_scanner in town, we're having dependency
resolution conflict because of flutter_markdown's tight dependency. This
patch loosens the dependency and resolves the conflict.

* Adds NineSliceSprite and optimizes gallery front page

Current numbers from my Nexus 5:
  "average_frame_build_time_millis": 22.6854,
  "missed_frame_build_budget_count": 15,
  "frame_count": 15

Filed https://github.com/flutter/flutter/issues/3296

@yjbanov @hansmuller @vlidholt

...by adding tests to our examples that don't import flutter_test, which
pins the relevant dependencies.

Also, provide more information when complaining about leaked transient
callbacks in tests.

Also, make tests display full information when they have an exception,
by bypassing the throttling we have for Android logging in tests.

Also, make the word wrapping not wrap stack traces if they happen to
be included in exception output.

Also, fix a leaked transient callback in the checkbox code.