Promote `obj` to non-null when checking `obj?.field == value`

[rrousselGit]

Consider the following code:

void fn() {
  Class? obj = ...;
  if (obj?.field == 'value') {
    obj.method();
  }
}

Currently, this fails and requires to instead write:

if (obj != null && obj.field == 'value') {
  obj.method();
}

Could we consider obj?.field == <non-nullable expression> to be equivalent to obj != null && obj.field == <expr>?

[rrousselGit]

One exception would be extensions on nullable objects. The following shoudn't work:

extension on Object? {
  Object? get field;
}

if (obj?.field == 'value') {
  // `obj` shouldn't be promoted to non-null, as `null.field` is possible
}

[mateusfccp]

Seems like a duplicate of #1224.

[rrousselGit]

Seems like it