* Teach flutter_tools to populate PUB_ENVIRONMENT

Will allow telemetry reporting on pub.dartlang.org once
flutter moves to 1.23.0-dev.10.0

* review changes

* improve the logger ui a bit for elapsed times

* invert param

* update daemon.dart

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions

* make showElapsedTime default to true

* support nested progresses

* improve hot reload message

* rethrow

Fixes #6417

* suppress warning; decouple the analyzer versions

* also pass in --no-packages-dir

* 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.

* use vendored dart sdk; check the expected version of flutter

* search up directories looking for the flutter root

* have flutter precache print if there's no work to do

* tweak precache

This will help folks run pub get after they upgrade their flutter.git
revision.

* use --no-precompile

* use the flutter bin cache's sdk

Also, resolve our package conflict, since reflectable has been fixed.

Now that we don't require the Dart SDK to be in your path, it's hard to run
./dev/update_packages.dart. Instead, you can now run `flutter update-packages`.

Fixes #1906

This removes a step that can cause trouble.

Fixes #1904