anchor.diff:   * implementation will be considerably more space- and time-efficient than
    * the default [List] implementation.
    */
  -class Float64List implements List<double>, ByteArrayViewable {
  +abstract class Float64List implements List<double>, ByteArrayViewable {
     /**
      * Creates a [Float64List] of the specified length (in elements), all of
      * whose elements are initially zero.
      */
  -  external Float64List(int length);
  +  external factory Float64List(int length);
   
     /**
      * Creates a [Float64List] _view_ of the specified region in the specified
  
positive.code:    * region does not contain an integral number of "int64s," or if it
     * is not "int64-aligned."
     */
    external Int64List.view(ByteArray array, [int start, int length]);
  }
  
  
  
negative.code:  * implementation will be considerably more space- and time-efficient than
   * the default [List] implementation.
   */
  abstract class Float32List implements List<double>, ByteArrayViewable {
    /**
     * Creates a [Float32List] of the specified length (in elements), all of
     * whose elements are initially zero.
     */
    external factory Float32List(int length);
  
    /**
     * Creates a [Float32List] _view_ of the specified region in the specified
  