Fix various warnings from the analyzer.

sdk/lib/utf/utf16.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.utf;
 
 /**
  * Decodes the UTF-16 bytes as an iterable. Thus, the consumer can only convert
  * as much of the input as needed. Determines the byte order from the BOM,
  * or uses big-endian as a default. This method always strips a leading BOM.
@@ skipping 188 matching lines @@
   Utf16CodeUnitDecoder get iterator =>
       new Utf16CodeUnitDecoder.fromListRangeIterator(codeunitsProvider(),
           replacementCodepoint);
 }
 
 /**
  * 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.
  */
-class Utf16BytesToCodeUnitsDecoder implements _ListRangeIterator {
+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]) {
@@ skipping 108 matching lines @@
   }
 
   int decode() {
     utf16EncodedBytesIterator.moveNext();
     int lo = utf16EncodedBytesIterator.current;
     utf16EncodedBytesIterator.moveNext();
     int hi = utf16EncodedBytesIterator.current;
     return (hi << 8) + lo;
   }
 }