Barry tells Joe that Snart has gone and that Oliver Queen told him that Snart left with some "friends of theirs on a trip." He is referring to DC's Legends of Tomorrow (2016), a spin-off series of Arrow (2012) and The Flash (2014), confirming that Barry knew about the superhero team through Oliver.
Dr. Wells mentions that the Earth-2 Mirror Master (Evan McCulloch) uses a mirror gun. This is a standard weapon used by the comic book version of the character.
Sam Scudder/Mirror Master appeared on The Flash (1990) in the episode "Done with Mirrors (1991)". He was played by David Cassidy, whose daughter, Katie Cassidy, played Laurel Lance/Black Canary/Black Siren in Arrow (2012).
This episode marks the 50th episode of the Flash since its inception in 2014.