This ensures that accidental usages of dart:io's
file API don't creep in over time.

Previously, overall status was reported as good if Xcode and homebrew
were installed, even if there were issues with either of the tools. We
now report partial status if we detect issues with either installation.

libimobiledevice 1.2.0 is incompatible with Xcode >= 8.0. Since the
libimobiledevice tools don't include a --version flag, check for valid
exit code on running idevicename, if an iOS device is connected.

* show device list in flutter doctor output
fixes https://github.com/flutter/flutter/issues/5625