Returns the number of objects in this list.
The valid indices for a list are 0 through length - 1.
int get length => JS("int", "#.length", this);Changes the length of this list.
If newLength is greater than
the current length, entries are initialized to null.
Throws an UnsupportedError if the list is fixed-length.
set length(int value) {
  throw new UnsupportedError("Cannot resize immutable List.");
}