This changes the DocSetPlatformFamily key to be "dartlang" instead of the name of the package (usually "flutter"). This is so that the IntelliJ plugin for Dash will be able to go directly to the docs for a symbol from a keystroke, instead of needing to search all the docsets each time.

Without this, flutter isn't part of the list of package names it searches. After this, it finds the flutter docs because they're declared to be part of the "dartlang" family of docs.

Dashing doesn't have a way to configure this, so we modify the Info.plist directly to make the change.