To view the list of file types recognized by WebStorm, go to Settings/Preferences (Ctrl+Alt+S) . If a file in your project is marked with the icon, it indicates that WebStorm can't recognize it. In this case, you can register and configure a new file type.
Icons in tool windows and structure views
Function in Dart
Anonymous bookmark – a check sign bookmark. Bookmarks are used for quick navigation within a file or across the entire project.
Lettered mnemonic bookmark – a letter in the interval from A to Z.
Numbered mnemonic bookmark – a number in the interval from 0 to 9.