From the field's description [1]:
DualStack previously enabled RFC 6555 Fast Fallback
support, also known as "Happy Eyeballs", in which IPv4 is
tried soon if IPv6 appears to be misconfigured and
hanging.
Deprecated: Fast Fallback is enabled by default. To
disable, set FallbackDelay to a negative value.
This field was deprecated in efc185029b,
which is included in Go 1.12beta1 and up.
[1]: 2ebe77a2fd/src/net/dial.go (L54-L61)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>