anchor.diff:  
     void insert(int index, E element) {
       ArgumentError.checkNotNull(index, "index");
  +    var length = this.length;
       RangeError.checkValueInInterval(index, 0, length, "index");
  -    if (index == this.length) {
  -      add(element);
  -      return;
  -    }
  -    this.length++;
  -    setRange(index + 1, this.length, this, index);
  -    this[index] = element;
  +    add(element);
  +    if (index != length) {
  +      setRange(index + 1, length + 1, this, index);
  +      this[index] = element;
  +    }
     }
   
     E removeAt(int index) {
  
positive.code: /// `ListBase` can be used as a base class for implementing the `List`
  /// interface.
  ///
  /// All operations are defined in terms of `length`, `operator[]`,
  /// `operator[]=` and `length=`, which need to be implemented.
  ///
  /// *NOTICE*: Forwarding just these four operations to a normal growable [List]
  /// (as created by `[]`) will give very bad performance for `add` and
  /// `addAll` operations of `ListBase`. These operations are implemented by
  /// increasing the length of the list by one for each `add` operation, and
  /// repeatedly increasing the length of a growable list is not efficient.
  /// To avoid this, either override 'add' and 'addAll' to also forward directly
  /// to the growable list, or, preferably, use `DelegatingList` from
  /// "package:collection/collection.dart" instead.
  abstract class ListBase<E> extends Object with ListMixin<E> {
    /// Converts a [List] to a [String].
    ///
  
negative.code: 
    // List interface.
    void add(E element) {
      // This implementation only works for lists which allow `null` as element.
      this[this.length++] = element;
    }
  
  