don't precompile dependencies when building the flutter tool

[jakemac53]

Description

Pub by default creates a snapshot of all executables from all direct dependencies on pub get/upgrade which is slow.

To make things worse, since flutter_tool pins all transitive deps that means it compiles all transitive executables since they all look like immediate deps.

Passing --no-precompile speeds up the pub get/upgrade step for flutter tool significantly by skipping this precompile step.

Timing breakdown

Without --no-precompile:

real 0m20.911s
user 1m38.711s
sys 0m4.446s

With --no-precompile:

real 0m6.257s
user 0m1.756s
sys 0m0.167s

These differences will be a lot more pronounced on machines with fewer cores as well.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[jonahwilliams]

LGTM!!!!!