index.html

<canvas id="myCanvas" width="500" height="500"></canvas>

main.dart

void main() {
  List<int> tableau = [1, 10, -5, 3, 4, -8, 2];
  int idx = 0;
  try
  {
  	idx = tableau.indexOf(tableau.firstWhere((i)=>i<0));
  }
  catch (e)
  {
    idx = -1;
  }
  print("index = $idx");
}

style.css

p {
  color: #888;
}