* Fix `flutter upgrade`

* Fix builds on iOS

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

* search up directories looking for the flutter root

* avoid sending analytics on ci systems

* review comments

* more terse flutter upgrade

* fix an issue when updating and the engine update code has changed

* call flutter precache; add a --no-color hidden option

* fix a lint related to getters/setters

* move the engine download code to dart

* fix populating the bin/cache/pkg dir

Now we run `flutter --version` to update our local cache of packages before
running `pub upgrade`, which reads from that cache.

Fixes #2953

rename to printTrace(), printStatus(), and printError()

Fixes #1084

1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

This command upgrades the version of flutter that you're using and runs
`pub upgrade` to update the transitive dependencies.