Review flutter attach performance

[xster]

On a simple test, it takes ~8s to connect to an already running flutter observatory via flutter attach. Though ~3s was from cleaning up the devfs and closing the process.

Log gist at https://gist.github.com/xster/5becc4937c2cef8d5e84660d0601abd1

There's a 2397ms apparently pure Dart part in the middle of the process too.

We should optimize.

[jonahwilliams]

It's initializing the frontend server and compiling the application code. This process is bootstrapped from the already compiled app if available.

[xster]

Ah, that sounds reasonable. You're saying that middle 2s+ would be drastically reduced in profile mode right?

[jonahwilliams]

Yeah, under profile mode we wouldn't attempt to compile anything - just connect

[jonahwilliams]

there also shouldn't be any devfs work in profile mode - so if that is case there is definitely a bug

[zanderso]

@xster @jonahwilliams were either of you able to verify whether extra work was being done in profile mode?

[xster]

It was not. The purpose of flutter attach is to connect to a previously detached full-flutter app or to an add-to-app module in debug rather than profile mode.

[zanderso]

Is there anything actionable left in this issue, or should we close it?

[xster]

I think we should profile whether any of the 8s spent during the attach can be improved on. If not, we should provide clearer output on what the tool is doing while the user's waiting.

[jonahwilliams]

@xster would you be willing to try this again? I've landed a few optimizations to devFS initialization which should help. Preferably one that is uncached and one that is fully cached