flutter#30153: Allow disabling experimental commands, devices on stable branch

flutter#30428: Update repair command for Arch Linux

Further, the IDE plugins for Flutter have had a number of updates since the last stable release of Flutter.

Visual Studio Code: February 21, 2019 (2.23.1)

Visual Studio Code: February 27, 2019 (2.24.0)

IntelliJ/Android Studio: March 29, 2019 (M34)

Visual Studio Code: April 17, 2019 (2.25.1)

IntelliJ/Android Studio: April 26, 2019 (M35)

Visual Studio Code: May 1, 2019 (2.26.1)

Dynamic Update (aka Code Push)