FutureGroup<T> constructor Null safety

1. Flutter
2. async
3. FutureGroup<T>
4. FutureGroup constructor

FutureGroup class

1. Constructors
2. FutureGroup
3. Properties
4. future
5. hashCode
6. isClosed
7. isIdle
8. onIdle
9. runtimeType
10. Methods
11. add
12. close
13. noSuchMethod
14. toString
15. Operators
16. operator ==