Proposal: argument passed by reference

There is a workaround for local variables (wrap in class) but it's not clear how to handle class fields. Amount of code to do routine stuff like copy values and check for changes is overwhelming comparing to languages like C++, C#, Delphi, Swift. It's just annoying to write and maintain.

```
// model

class DateRange {
DateTime start, end;
...
}

// utility

/// returns false if not changed
bool editDate(ref DateTime date) {
...
}

// view

DateRange range;
...
btn1.onPressed = () {
if(editDate(range.start))
save();
};
btn2.onPressed = () {
if(editDate(range.end))
save();
};
```

Compare to
```
btn1.onPressed = () {
final out = editDate(range.start);
if(out != null && out != range.start) {
range.start = out;
save();
}
};
```
This is two lines vs. five. And in our business logic it's a very common thing.

No files, tests, or entry points are named. Start by reviewing the proposal and its Dart examples, then determine the intended semantics and language-design constraints for reference arguments, including class fields. Done would require an accepted, specified design rather than a localized code change.

Written by the indexing model from the issue text.