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

[munificent]

Moved from: dart-lang/dart_style#117

Instead of:

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

prefer:

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

[pq]

Migrated (back) to github: https://github.com/dart-lang/dart_lint/issues/4

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Added Invalid label.