NNBD: How to increment/decrement a nullable expression using the operators ++/--?

[simphotonics]

What is the syntax for incrementing/decrementing a nullable expression (such as the value of a map entry) using the prefix/postfix operators ++ or -- ? I am getting the error shown below.

$ dart --version
Dart SDK version: 2.9.3 (stable) (Unknown timestamp) on "linux_x64"

final map = <int, int>{};
var index = 0;
...

if (map[index] == null){
  map[index] = 1;
}else{
  map[index] = map[index]! + 1;  // This works.
   ++map[index]!; // Missing selector such as '.identifier' or '[0]'. Try adding a selector.dart(missing_assignable_selector)
   ++(map[index]!); // Missing selector such as '.identifier' or '[0]'. Try adding a selector.dart(missing_assignable_selector)
   map[index]! += 1; // Missing selector such as '.identifier' or '[0]'. Try adding a selector.dart(missing_assignable_selector)
}

It seems that the expression map[index]! is not assignable.

[eernstg]

This discussion has been raised in #1113. There is currently no mechanism which will allow you to combine null checks (!) with increment/decrement, but please continue the discussion there.

[simphotonics]

Thank you for pointing out #1113.