'is' does not cast variables out of the scope

[nosmirck]

When using is casting should work with variables out of the scope too. Here is an example:

final A b = B(42);

void main() {
  //This one doesn't allow compilation because
  //"The getter 'i' isn't defined for the type A" even though
  //the variable has been casted inside the `if (b is B)`
  if (b is B) {
    print(b.i);
  }

  //Making a local scope reference of this variable works as expected.
  final _b = b;
  if (_b is B) {
    print(_b.i);
  }
}

abstract class A {}

class B extends A {
  final int i;
  B(this.i);
}

Tested this on the latest DartPad with null safety enabled.

[leafpetersen]

This is a general instance of this issue. We could promote final global variables (as opposed to fields), but only at the expense of breaking the symmetry between variables and getter/setter pairs. See here for a list of issues discussing possible approaches to making this easier to work with.