Bump dart-lang/setup-dart from 1.5.1 to 1.6.0 #215

Job	Run time
analyze (dev)	23s
test_dart (ubuntu-latest, 3.0.0)	2m 55s
test_dart (ubuntu-latest, dev)	2m 48s
test_dart (macos-latest, 3.0.0)	3m 49s
test_dart (macos-latest, dev)	4m 47s
test_dart (windows-latest, 3.0.0)	3m 21s
test_flutter (ubuntu-latest, beta)	3m 21s
test_dart (windows-latest, dev)	4m 35s
test_flutter (ubuntu-latest, stable)	3m 4s
test_flutter (macos-latest, beta)	3m 24s
test_flutter (macos-latest, stable)	4m 29s
test_flutter (windows-latest, beta)	5m 11s
test_flutter (windows-latest, stable)	5m 42s
	47m 49s

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