Use ";" instead of "{}" for empty constructor bodies

[pq]

Migrated from dartbug.com/22004:

Instead of:

class Point {
  int x, y;
  Point(this.x, this.y) {}
}

prefer:

class Point {
  int x, y;
  Point(this.x, this.y);
}

[pq]

Duplicated by: #57158