Bump dart-lang/setup-dart from 1.6.2 to 1.6.4 #263

Job	Run time
analyze (dev)	16s
test_dart (ubuntu-latest, 3.0.0)	2m 7s
test_dart (ubuntu-latest, dev)	1m 14s
test_dart (macos-latest, 3.0.0)	1m 21s
test_flutter (ubuntu-latest, beta)	1m 44s
test_dart (macos-latest, dev)	1m 20s
test_flutter (ubuntu-latest, stable)	1m 52s
test_dart (windows-latest, 3.0.0)	2m 15s
test_flutter (macos-latest, beta)	1m 36s
test_dart (windows-latest, dev)	2m 38s
test_flutter (macos-latest, stable)	1m 49s
test_flutter (windows-latest, beta)	4m 13s
test_flutter (windows-latest, stable)	3m 8s
	25m 33s

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