Convert UTF-16 encoded bytes to UTF-16 code units by grouping 1-2 bytes to produce the code unit (0-(2^16)-1). Relies on BOM to determine endian-ness, and defaults to BE.

abstract class Utf16BytesToCodeUnitsDecoder implements _ListRangeIterator {
 final _ListRangeIterator utf16EncodedBytesIterator;
 final int replacementCodepoint;
 int _current = null;

 Utf16BytesToCodeUnitsDecoder._fromListRangeIterator(
     this.utf16EncodedBytesIterator, this.replacementCodepoint);

 factory Utf16BytesToCodeUnitsDecoder(List<int> utf16EncodedBytes, [
     int offset = 0, int length,
     int replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) {
   if (length == null) {
     length = utf16EncodedBytes.length - offset;
   }
   if (hasUtf16beBom(utf16EncodedBytes, offset, length)) {
     return new Utf16beBytesToCodeUnitsDecoder(utf16EncodedBytes, offset + 2,
         length - 2, false, replacementCodepoint);
   } else if (hasUtf16leBom(utf16EncodedBytes, offset, length)) {
     return new Utf16leBytesToCodeUnitsDecoder(utf16EncodedBytes, offset + 2,
         length - 2, false, replacementCodepoint);
   } else {
     return new Utf16beBytesToCodeUnitsDecoder(utf16EncodedBytes, offset,
         length, false, replacementCodepoint);
   }
 }

 /**
  * Provides a fast way to decode the rest of the source bytes in a single
  * call. This method trades memory for improved speed in that it potentially
  * over-allocates the List containing results.
  */
 List<int> decodeRest() {
   List<int> codeunits = new List<int>(remaining);
   int i = 0;
   while (moveNext()) {
     codeunits[i++] = current;
   }
   if (i == codeunits.length) {
     return codeunits;
   } else {
     List<int> truncCodeunits = new List<int>(i);
     truncCodeunits.setRange(0, i, codeunits);
     return truncCodeunits;
   }
 }

 int get current => _current;

 bool moveNext() {
   _current = null;
   if (utf16EncodedBytesIterator.remaining < 2) {
     utf16EncodedBytesIterator.moveNext();
     if (replacementCodepoint != null) {
       _current = replacementCodepoint;
       return true;
     } else {
       throw new ArgumentError(
           "Invalid UTF16 at ${utf16EncodedBytesIterator.position}");
     }
   } else {
     _current = decode();
     return true;
   }
 }

 int get position => utf16EncodedBytesIterator.position ~/ 2;

 void backup([int by = 1]) {
   utf16EncodedBytesIterator.backup(2 * by);
 }

 int get remaining => (utf16EncodedBytesIterator.remaining + 1) ~/ 2;

 void skip([int count = 1]) {
   utf16EncodedBytesIterator.skip(2 * count);
 }

 int decode();
}