Make compiler optional in calls to currentPlatform

[natebosch]

Allow backwards compatibility with current callers. This might make it
easier to land flutter/engine#46592

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:test_core	0.5.8	ready to publish	test_core-v0.5.8
package:test	1.24.8	ready to publish	test-v1.24.8
package:checks	0.3.0-wip	WIP (no publish necessary)
package:test_api	0.6.2-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.