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.

flatbuffers/dart/test/flat_buffers_test.dart

Lines 818 to 827 in 7106d86

	// TODO currently broken because of struct builder issue, see #6688
	// final monster2 = example.Monster(data); // Monster (reader)
	// expect(
	// // map Monster => MonsterT, Vec3 => Vec3T, ...
	// monster2.toString().replaceAllMapped(
	// RegExp('([a-zA-z0-9]+){'), (match) => match.group(1) + 'T{'),
	// monster.toString());
	//
	// final monster3 = monster2.unpack(); // MonsterT
	// expect(monster3.toString(), monster.toString());

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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! :)