Make convert library strong-mode compliant.

sdk/lib/convert/latin1.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 [Latin1Codec].
  *
  * This instance provides a convenient access to the most common ISO Latin 1
@@ skipping 41 matching lines @@
    */
   String decode(List<int> bytes, { bool allowInvalid }) {
     if (allowInvalid == null) allowInvalid = _allowInvalid;
     if (allowInvalid) {
       return const Latin1Decoder(allowInvalid: true).convert(bytes);
     } else {
       return const Latin1Decoder(allowInvalid: false).convert(bytes);
     }
   }
 
-  Converter<String, List<int>> get encoder => const Latin1Encoder();
+  Latin1Encoder get encoder => const Latin1Encoder();
 
-  Converter<List<int>, String> get decoder =>
+  Latin1Decoder get decoder =>
       _allowInvalid ? const Latin1Decoder(allowInvalid: true)
                     : const Latin1Decoder(allowInvalid: false);
 }
 
 /**
  * This class converts strings of only ISO Latin-1 characters to bytes.
  */
 class Latin1Encoder extends _UnicodeSubsetEncoder {
   const Latin1Encoder() : super(_LATIN1_MASK);
 }
@@ skipping 110 matching lines @@
       }
     }
     if (start < end) {
       _addSliceToSink(source, start, end, isLast);
     }
     if (isLast) {
       close();
     }
   }
 }