Make Utf8Decoder and Utf8Encoder's convert methods take start and end too.

tests/lib/convert/latin1_test.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.
 
 import "package:expect/expect.dart";
 import 'dart:convert';
 
 var latin1Strings = [
   "pure ascii",
   "blåbærgrød",
@@ skipping 34 matching lines @@
       Expect.equals(latin1String, roundTripString);
       roundTripString = codec.decode(bytes);
       Expect.equals(latin1String, roundTripString);
     }
 
     for (var nonLatin1String in nonLatin1Strings) {
       Expect.throws(() {
         print(codec.encoder.convert(nonLatin1String));
       }, null, nonLatin1String);
     }
+
+    var encode = codec.encoder.convert;
+    Expect.listEquals([0x42, 0xff, 0x44], encode("AB\xffDE", 1, 4));
+    Expect.listEquals([0x42, 0xff, 0x44, 0x45], encode("AB\xffDE", 1));
+    Expect.listEquals([0x42, 0xff, 0x44], encode("\u3000B\xffD\u3000", 1, 4));
+    Expect.throws(() { encode("\u3000B\xffD\u3000", 0, 4); });
+    Expect.throws(() { encode("\u3000B\xffD\u3000", 1); });
+    Expect.throws(() { encode("\u3000B\xffD\u3000", 1, 5); });
+    Expect.throws(() { encode("\u3000B\xffD\u3000", -1, 4); });
+    Expect.throws(() { encode("\u3000B\xffD\u3000", 1, -1); });
+    Expect.throws(() { encode("\u3000B\xffD\u3000", 3, 2); });
+
+    var decode = codec.decoder.convert;
+    Expect.equals("B\xffD", decode([0x41, 0x42, 0xff, 0x44, 0x45], 1, 4));
+    Expect.equals("B\xffDE", decode([0x41, 0x42, 0xff, 0x44, 0x45], 1));
+    Expect.equals("B\xffD", decode([0xFF, 0x42, 0xff, 0x44, 0xFF], 1, 4));
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], 0, 4); });
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], 1); });
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], 1, 5); });
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], -1, 4); });
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], 1, -1); });
+    Expect.throws(() { decode([0x3000, 0x42, 0xff, 0x44, 0x3000], 3, 2); });
   }
 
   var allowInvalidCodec = new Latin1Codec(allowInvalid: true);
   var invalidBytes = [0, 1, 0xff, 0xdead, 0];
   String decoded = allowInvalidCodec.decode(invalidBytes);
   Expect.equals("\x00\x01\xFF\uFFFD\x00", decoded);
   decoded = allowInvalidCodec.decoder.convert(invalidBytes);
   Expect.equals("\x00\x01\xFF\uFFFD\x00", decoded);
   decoded = LATIN1.decode(invalidBytes, allowInvalid: true);
   Expect.equals("\x00\x01\xFF\uFFFD\x00", decoded);
@@ skipping 62 matching lines @@
       }
       for (var nonLatin1String in nonLatin1Strings) {
         var units = nonLatin1String.codeUnits.toList();
         Expect.throws(() {
           decode(units, chunkSize, converter);
         });
       }
     }
   }
 }