Bump dart-lang/setup-dart from 1.6.2 to 1.6.4 #443

Job	Run time
build (ubuntu, stable, native_assets_builder, false)	3m 6s
build (ubuntu, stable, native_assets_cli, false)	2m 10s
build (ubuntu, stable, native_toolchain_c, false)	1m 50s
build (ubuntu, dev, native_assets_builder, false)	23s
build (ubuntu, dev, native_assets_cli, false)	21s
build (ubuntu, dev, native_toolchain_c, false)	17s
build (macos, stable, native_assets_builder, false)	18s
build (macos, stable, native_assets_cli, false)	18s
build (macos, stable, native_toolchain_c, false)	19s
build (windows, stable, native_assets_builder, false)	5m 27s
build (windows, stable, native_assets_cli, false)	3m 11s
build (windows, stable, native_toolchain_c, false)	4m 10s
coverage-finished	0s
	21m 50s

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