Request: calling a function only when a parameter is not null.

It's awkward to call a function when a parameter is not null, such as:

```dart
String toPrint;

if (toPrint != null) {
print(toPrint);
}
```

This is significantly more work than calling a method when the target is not null:

```dart
Printer printer;

printer?.print(toPrint);
```

So naturally we developers have gotten privileged and want both :)