Flight Master's Whistle
Source: Quest. Did you lose this handy little whistle? Talk to in the Violet Citadel, Dalaran to get another (In Legion).
The 
Flight Master's Whistle is a Legion toy earned through completing the Uniting the Isles quest, which requires reaching Friendly reputation with the Broken Isles factions.
Guide awaiting review
Reach Friendly Broken Isles Reputation
Work toward Friendly reputation with the Broken Isles factions to unlock the Uniting the Isles quest, which rewards the Flight Master's Whistle.
Complete Uniting the Isles Quest
Turn in the Uniting the Isles quest to receive your Flight Master's Whistle. Archmage Khadgar can be found at 58.43, 62.67 in the Violet Citadel, Dalaran, and will provide a replacement whistle if you ever lose or delete yours.
Retrieve a Replacement Whistle
If you deleted or lost your whistle during Battle for Azeroth content, Halford Wyrmbane or Nathanos Blightcaller will provide a fresh Flight Master's Whistle when you unlock World Quests in Zandalar or Kul Tiras at max level.
Tips
The whistle only works with already-discovered flight paths in your current zone — undiscovered flight points will not be used, and the whistle will send you to the nearest known one instead.
Near zone borders, the whistle may send you to a flight point in an adjacent zone if your nearest known flight path belongs to that zone, so be mindful of your position when using it.
