Treat member access of a constant record as a constant expression.

Since a record's type is known at compile time, and if a record is constant then all of its members are constant, then we should just be able to use a constant record's members as a constant as well in my opinion.

```dart
const dimensions = (width: 250, height: 750);
const child = SizedBox(width: dimensions.width, height: dimensions.height);
```