Example of a possible extension of List to include a indexesOf method.

ListExtension.dart

class ListExtension  {
  /**
  * Returns a [List] of indexes of the given [element], starting 
  * the search at index [startIndex] to [endIndex] (exclusive).
  * Returns an empty [List] if [element] is not found.
  */
  static List<num> indexesOf(List list, Object element, int startIndex, int endIndex) {
    List<num> n = new List<num>();
    
    if (startIndex >= list.length) {
      return n;
    }
    
    if (startIndex < 0) {
      startIndex = 0;
    }
    
    for (int i = startIndex; i < endIndex; i++) {
      if (list[i] == element) {
        n.add(i);
      }
    }
    
    return n;   
  }
}

run.dart

void main() {
  void run() {
    List<num> l = new List<num>();
    l.add(10);
    l.add(20);
    l.add(10);
    l.add(30);
    
    List<num> e = ListExtension.indexesOf(l, 10, 0, l.length);
    
    Expect.equals(2, e.length);
    Expect.equals(0, e[0]);
    Expect.equals(2, e[1]);
   
  }
  run();
}