Statements inside if conditions

Statements inside control statement conditions to store it, check it and also scope it inside the control statement block would be very useful.

```dart
if((Point point = parseCursorPosition(chars)) != null) {
// Do something with point
}
```
or

```dart
if(Point point = parseCursorPosition(chars); point != null) {
// Do something with point
}
```
This feature is very popular in golang world.

Right now, this is possible:

```dart
Point point;
if((point = parseCursorPosition(chars)) != null) {
// Do something with point
}
```
The only problem is this leaks the point outside its scope.