Skip linux_fuchsia on presubmit except when changing fuchsia code

[jmagman]

Only run Fuschia tests in presubmit when CI or shell/platform/fuchsia/ files change.

[jmagman]

I didn't remove the commented out runIfNot code, since flutter/flutter#138559 is being investigated. I can do so though if @matanlurey prefers.

[matanlurey]

I don't think this change is safe, unfortunately - if we change something that Fuchsia depends on we'll get a red tree.

If we're seeing serious latency or flaky problems though, perhaps we could decide this is worth it anyway. What's the rationale here?

[jmagman]

What's the rationale here?

I should have filled out the description more thoroughly. I'm always seeing this run when I'm only touching the iOS embedder which would definitely not impact Fuchsia. However this change is way too aggressive:

In flutter/engine#47994, I accidentally broke the Fuchsia build, despite all of the checks passing (and using the new test: all check to additionally run post-submits as pre-submits). It was reverted in flutter/engine#48130.

I will close this in favor of getting runIfNot working with shell/platform/darwin/** flutter/flutter#138559

[matanlurey]

I will close this in favor of getting runIfNot working with shell/platform/darwin/** flutter/flutter#138559

I think runIfNot was straight up deleted because it didn't work as folks expect. I should delete the remains.