Fix various warnings from the analyzer.

sdk/lib/utf/utf32.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-32 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 177 matching lines @@
   final Utf32BytesDecoderProvider codeunitsProvider;
 
   IterableUtf32Decoder._(this.codeunitsProvider);
 
   Utf32BytesDecoder get iterator => codeunitsProvider();
 }
 
 /**
  * Abstrace parent class converts encoded bytes to codepoints.
  */
-class Utf32BytesDecoder implements _ListRangeIterator {
+abstract class Utf32BytesDecoder implements _ListRangeIterator {
   final _ListRangeIterator utf32EncodedBytesIterator;
   final int replacementCodepoint;
   int _current = null;
 
   Utf32BytesDecoder._fromListRangeIterator(
       this.utf32EncodedBytesIterator, this.replacementCodepoint);
 
   factory Utf32BytesDecoder(List<int> utf32EncodedBytes, [
       int offset = 0, int length,
       int replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) {
@@ skipping 120 matching lines @@
     value += (utf32EncodedBytesIterator.current << 24);
     return value;
   }
 }
 
 bool _validCodepoint(int codepoint) {
   return (codepoint >= 0 && codepoint < UNICODE_UTF16_RESERVED_LO) ||
       (codepoint > UNICODE_UTF16_RESERVED_HI &&
       codepoint < UNICODE_VALID_RANGE_MAX);
 }