dart: Fix incorrect write in Float64 write method

[NotTsunami]

Closes #8070, closes #6688.

Credits @Llamadmiral for pointing out the fix and @vaind for the test case.

NOTE: The currently commented out test code is still commented out as it isn't a great test case. The test will fail because no ColorT type exists (as expected). We should probably get a better test case. https://github.com/google/flatbuffers/blob/7106d86685e3395efc6df5d2ee5914fe622ed6b9/dart/test/flat_buffers_test.dart#L818-L827

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[NotTsunami]

Hi Llamadmiral, I attributed the commit to you because you have found the fix for it. For this to get merged with your handle in the credit, you need to sign the CLA!

[Llamadmiral]

Hello @NotTsunami,

Thanks for attributing the commit to me! I signed the CLA, and pressed rescan, I think it finished successfully.

Thanks again! :)