The Supergirl/Flash crossover was everything a geek would want in seeing two super-heroes meet for the first time. There were confused introductions, gleeful banter in one hero being around another super-hero, and teamwork. Neither the Flash nor Supergirl saw the other as a threat. It’s like when geeks meet at a comic convention and discuss the comics they love.
This episode was a much-needed antidote after Batman v Superman. Supergirl and Flash could have broke-out in song and it would have been heartwarming. If the writers ever need a reason for Supergirl to sing, just pull from Final Crisis with Superman singing Darkseid out of existence. Perhaps a musical vibration that only Kara can hit could open a portal to Earth-1 for an annual cross-over.
The Flash did more than just help Supergirl stop Livewire and Silver Banshee; Barry Allen gave National City the technology to safely arrest super-villains by suppressing their superpowers. This most likely was The Boot rifle seen in The Flash.
This very short dialog means that the DEO will no longer be holding metahumans in a secret prison without a trial. Supergirl’s villains have been held in major violations of their Civil Rights, including:
The writ of habeas corpus, which requires a person in custody to be brought before a Court;
The 4th Amendment, which protects people from arbitrary arrests;
The 5th Amendment, which protects people from being deprived of life, liberty, or property, without due process of law;
The 6th Amendment right to counsel;
The 7th Amendment right to a trial; and
The 8th Amendment prohibition against cruel and unusual punishment.
Holding human beings held prisoner in solitary confinement, in glass cells, without a bed or bathroom, without access to a lawyer, or even a trial, screams so many Civil Rights violations that not even Silver Banshee could hit all the notes. Yes, Earth-Supergirl did not have any facilities set-up to handle such individuals, but thanks to Barry Allen, our heroes will not be aiding in massive Constitutional violations after saving the day.