[fixnum] update the min. required dart sdk

[devoncarew]

• update the min. required dart sdk to 3.4
• related to Run fixnum tests with dart2wasm #906

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Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
fixnum	Breaking	1.1.1	1.2.0-wip	2.0.0 Got "1.2.0-wip" expected >= "2.0.0" (breaking changes)	⚠️

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[osa1]

FWIW The package still works with 3.1 (tests pass), you just can't test it with dart2wasm because of a dart2wasm or package:test issue (or maybe multiple issues).

It should still be OK to update as 3.4.0 is old enough.

[devoncarew]

FWIW The package still works with 3.1 (tests pass), you just can't test it with dart2wasm because of a dart2wasm or package:test issue (or maybe multiple issues).

It should still be OK to update as 3.4.0 is old enough.

Yup, agreed; it's not necessary to bump just for the tests, but 3.1 is pretty old, and keeping the CI + package deps in sync is useful.