if - assignment operator

At the moment Dart has ternary operator that requires "else" part.
In the cases when I need something like this
```
var s1 = 3;
s1 = s1 > 3 ? 100500 : s1
```

": s1" is redunant.

Or I should use "if"

```
var s1 = 3;
if(s1 > 3){
s1 = 100500;
}
```

I think It will be useful to have another one aproach to do this, something like this:

```
var s1 = 3;
s1 ?= s1 > 3 : 100500;
```

```
var s2 = 3;
s2 ?= s2 > 3 : 100500 + 56 * 78;
```

If the condition is false than all expression ignors. And the old value will not change.

This is something like collection "if" but witout collection.