Should implicit constructors be introduced at the same time of introduction of inline classes?

Without implicit constructors,
```dart
inline class MyInt(int _) implements int;

void main() {
var mi = MyInt(10);
mi = MyInt(mi * 2);
}
```

With implicit constructors,
```dart
inline class implicit MyInt(int _) implements int;

void main() {
MyInt mi = 10;
mi *= 2;
}
```

cf. #108 #309

Compare the two inline-class examples and the related issues #108 and #309. Determine whether implicit constructors should be introduced with inline classes, and define the desired syntax and behavior before implementation.

Written by the indexing model from the issue text.