`b3, b4 = 1, "foo"` fails on C++ and Dart
Case identified at https://github.com/adsharma/py2many/pull/363#discussion_r663622435 , but the failures are unrelated to that PR

C++ fails with
```
/Users/john.vandenberg/python/py2many/tests/build/coverage.cpp:41:12: error: 'b3' was not declared in this scope; did you mean 'b2'?
   41 |   std::tie(b3, b4) = std::make_tuple(1, std::string{"foo"});
      |            ^~
      |            b2
/Users/john.vandenberg/python/py2many/tests/build/coverage.cpp:41:16: error: 'b4' was not declared in this scope; did you mean 'b2'?
   41 |   std::tie(b3, b4) = std::make_tuple(1, std::string{"foo"});
      |                ^~
      |                b2
```

Dart fails with

```
coverage.dart:34:3: Error: Setter not found: 'b3'.
  b3 = __tmp1.item1;
  ^^
coverage.dart:35:3: Error: Setter not found: 'b4'.
  b4 = __tmp1.item2;
  ^^
coverage.dart:36:10: Error: Getter not found: 'b3'.
  assert(b3 == 1);
         ^^
coverage.dart:37:10: Error: Getter not found: 'b4'.
  assert(b4 == "foo");
         ^^
coverage.dart:33:38: Error: The argument type 'String' can't be assigned to the parameter type 'int'.
  final __tmp1 = Tuple2<int, int>(1, "foo");
                                     ^
```