anchor.diff:        {Reachability? reachable}) {
       reachable ??= this.reachable;
       Map<Variable?, VariableModel<Variable, Type>> newVariableInfo =
  -        new Map<Variable?, VariableModel<Variable, Type>>.from(variableInfo);
  +        new Map<Variable?, VariableModel<Variable, Type>>.of(variableInfo);
       reference.storeInfo(newVariableInfo, model);
       return new FlowModel<Variable, Type>.withInfo(reachable, newVariableInfo);
     }
  
positive.code:             ssaNode: null);
        } else if (!info.writeCaptured) {
          (newVariableInfo ??=
              new Map<Variable, VariableModel<Variable, Type>>.from(
                  variableInfo))[variable] = info.writeCapture();
        }
      }
  
negative.code:           _assignedVariables._anywhere._written.contains(entry.key)) {
          continue;
        }
        (newKnownTypes ??= new Map<Variable, Type>.of(_knownTypes))[entry.key] =
            entry.value;
      }
      if (newKnownTypes != null) _knownTypes = newKnownTypes;
  