Type-inference acts on dead code (multiple returns)

A method with multiple returns of different types returns `Object`, even if the second return is dead code and never gets executed.

```dart
void main() {
final a = (){
return "string";
return 1;
}();

final String b = a;
// Error: A value of type 'Object' can't be assigned to a variable of type 'String'
}
```

The expected behaviour is that the function should return `String` as if the second return wouldn't exist at all
```dart
void main() {
final a = (){
return "string";
// return 1;
}();

final String b = a;
}
```

Dart SDK 2.14.2

Start by reproducing the Dart example and tracing how the closure's return type is inferred from multiple returns. Check the language type-inference specification and the relevant implementation entry point; done means the unreachable second return no longer widens the inferred type, while reachable differing returns retain current behavior.

Written by the indexing model from the issue text.