Support loading systrace files into DevTools

[kenzieschmoll]

Performance measurements can be affected by both the overhead of tracing with the Dart VM as well as the overhead caused by connecting an application to DevTools. For those reasons, tracing to systrace is more performant. (Related issue regarding timeline recording format: dart-lang/sdk#47631). Systrace files cannot be loaded into DevTools currently, which means they must be loaded into other trace viewers like perfetto.

There are tools that convert systrace files into the Chrome trace event format that DevTools expects (@dnfield do you have links to any of these converters you mentioned?). We should find a way to use these tools to convert the trace (ideal), or at the very least, point users to how they can convert their traces for visualization in DevTools.

Bonus would be if we automated this on the command line so the workflow is as such:

• user runs a command to run/test something and adds a flag to trace to systrace
• at exit of run/test, the CLI tool converts the file to the Chrome trace event format that DevTools expects and provides a hotkey the user can press or a link they can visit to automatically load trace in DevTools.

CC @xster @dnfield @a-siva @jiahaog

[xster]

One nit clarification, at the "at exit of run/test", if the Dart program still running was not a requirement, it would be very convenient.