https://github.com/flutter/flutter/issues/17956
* Add a `debugInstrumentAction()` method that will wrap an action
in a stopwatch and print the time it took to run the action.
* Add a global `debugInstrumentationEnabled` that will control
whether `debugInstrumentAction()` does anything (even in debug
builds).
* Add some basic instrumentation to `registerServiceExtension()`
* Temporarily enable the `debugInstrumentationEnabled` flag in the
Gallery to give us better visibility into what's happening in
https://github.com/flutter/flutter/issues/17956
