Add flutter.el to additional packages #512

Job	Run time
test (ubuntu-latest, 26.3)	42s
test (ubuntu-latest, 27.1)	1m 10s
test (ubuntu-latest, 27.2)	1m 13s
test (ubuntu-latest, 28.1)	1m 24s
test (ubuntu-latest, snapshot)	1m 4s
test (macos-latest, 26.3)	1m 41s
test (macos-latest, 27.1)	2m 17s
test (macos-latest, 27.2)	2m 38s
test (macos-latest, 28.1)	2m 39s
test (macos-latest, snapshot)	2m 19s
test (windows-latest, 26.3)	2m 13s
test (windows-latest, 27.1)	2m 11s
test (windows-latest, 27.2)	1m 56s
test (windows-latest, 28.1)	2m 5s
test (windows-latest, snapshot)	34s
	26m 6s

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