When switching between channels, we were leaving around the version freshness stamp file (bin/cache/flutter_version_check.stamp), which meant that the flutter tool would read from that file to see what the cached date of the most recent commit to the current channel (branch) was. The problem was that since the file was created while on the previous channel, the cached date was for the wrong channel, so if you switch from master to beta, flutter would think that the channel was out of date, and a new version was available, at least for three days after the first time it checked (after three days since the last time the freshness was checked, the cached date would get updated).
This PR modifies the channel command to remove that stamp file whenever the user switches channels, so that the cached date will be from the right channel when it is recreated.
Fixes#21134
Generate the "version" file from git tags.
Remove the old VERSION file and mentions of versions in pubspec.yaml files.
Replace the old update_versions.dart script with a new roll_dev.dart script.
Update "flutter channel".
Update "flutter upgrade", including making it transition from alpha to dev.
Update "flutter --version" and "flutter doctor".
* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8b5
* 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
* refactor _run to runCmd
* replace requiresProjectRoot getter with call to commandValidator
* replace requiresDevice getter with call to findTargetDevice
* trace command requires a debug connection, not a device
* inline androidOnly getter
* rename command methods to verifyTheRunCmd and runCmd
* move common verification into BuildSubCommand
* rename deviceForCommand to device
* rename methods to verifyThenRunCommand and runCommand
