Add support for IOS_SIMULATOR_HOME environment variable in IOSSimulator.logFilePath
flutter_tools can be run on environments where the user's HOME directory
is not the root of the iOS simulators' configs. This change adds support
for such environments by allowing the caller to set the simulator root
directory via an environment variable.
1. Migrate simulator device log tailing to os_log toolchain
2. When the log tag (component) is available (iOS 11/Xcode 9), filter to
the set of log lines with tag 'Flutter'.
As of iOS 11 / Xcode 9, Flutter engine logs are no longer recorded in the
simulator's syslog file, which we previously read using tail -f. Instead
they're now accessible through Apple's new macOS/iOS os_log facility,
via /usr/bin/log, which supports a relatively flexible query language.
When run in non-interactive mode, /usr/bin/log buffers its output in 4k
chunks, which is significantly smaller than what's emitted up to the
point where the observatory/diagnostics port information is logged. As a
workaround we force it to run in interactive mode via the script tool.
