Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/ascii.dart

 // Copyright (c) 2013, 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.convert;
 
 /**
  * An instance of the default implementation of the [AsciiCodec].
  *
  * This instance provides a convenient access to the most common ASCII
@@ skipping 51 matching lines @@
 
   AsciiEncoder get encoder => const AsciiEncoder();
 
   AsciiDecoder get decoder =>
       _allowInvalid ? const AsciiDecoder(allowInvalid: true)
                     : const AsciiDecoder(allowInvalid: false);
 }
 
 // Superclass for [AsciiEncoder] and [Latin1Encoder].
 // Generalizes common operations that only differ by a mask;
-class _UnicodeSubsetEncoder extends Converter<String, List<int>> {
+class _UnicodeSubsetEncoder extends Converter<String, List<int>>
+    implements ChunkedConverter<String, List<int>, String, List<int>> {
+
   final int _subsetMask;
 
   const _UnicodeSubsetEncoder(this._subsetMask);
 
   /**
    * Converts the [String] into a list of its code units.
    *
    * If [start] and [end] are provided, only the substring
    * `string.substring(start, end)` is used as input to the conversion.
    */
@@ skipping 64 matching lines @@
     if (isLast) {
       close();
     }
   }
 }
 
 /**
  * This class converts Latin-1 bytes (lists of unsigned 8-bit integers)
  * to a string.
  */
-abstract class _UnicodeSubsetDecoder extends Converter<List<int>, String> {
+abstract class _UnicodeSubsetDecoder extends Converter<List<int>, String>
+    implements ChunkedConverter<List<int>, String, List<int>, String> {
+
   final bool _allowInvalid;
   final int _subsetMask;
 
   /**
    * Instantiates a new decoder.
    *
    * The [_allowInvalid] argument defines how [convert] deals
    * with invalid bytes.
    *
    * The [_subsetMask] argument is a bit mask used to define the subset
@@ skipping 133 matching lines @@
     RangeError.checkValidRange(start, end, length);
     if (start < end) {
       if (start != 0 || end != length) {
         source = source.sublist(start, end);
       }
       add(source);
     }
     if (isLast) close();
   }
 }