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

tests/lib/convert/ascii_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 asciiStrings = [
   "pure ascii",
   "\x00 with control characters \n",
@@ skipping 36 matching lines @@
       Expect.equals(asciiString, roundTripString);
       roundTripString = codec.decode(bytes);
       Expect.equals(asciiString, roundTripString);
     }
 
     for (var nonAsciiString in nonAsciiStrings) {
       Expect.throws(() {
         print(codec.encoder.convert(nonAsciiString));
       }, null, nonAsciiString);
     }
+
+    var encode = codec.encoder.convert;
+    Expect.listEquals([0x42, 0x43, 0x44], encode("ABCDE", 1, 4));
+    Expect.listEquals([0x42, 0x43, 0x44, 0x45], encode("ABCDE", 1));
+    Expect.listEquals([0x42, 0x43, 0x44], encode("\xffBCD\xff", 1, 4));
+    Expect.throws(() { encode("\xffBCD\xff", 0, 4); });
+    Expect.throws(() { encode("\xffBCD\xff", 1); });
+    Expect.throws(() { encode("\xffBCD\xff", 1, 5); });
+    Expect.throws(() { encode("\xffBCD\xff", -1, 4); });
+    Expect.throws(() { encode("\xffBCD\xff", 1, -1); });
+    Expect.throws(() { encode("\xffBCD\xff", 3, 2); });
+
+    var decode = codec.decoder.convert;
+    Expect.equals("BCD", decode([0x41, 0x42, 0x43, 0x44, 0x45], 1, 4));
+    Expect.equals("BCDE", decode([0x41, 0x42, 0x43, 0x44, 0x45], 1));
+    Expect.equals("BCD", decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 1, 4));
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 0, 4); });
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 1); });
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 1, 5); });
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], -1, 4); });
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 1, -1); });
+    Expect.throws(() { decode([0xFF, 0x42, 0x43, 0x44, 0xFF], 3, 2); });
   }
 
   var allowInvalidCodec = new AsciiCodec(allowInvalid: true);
   var invalidBytes = [0, 1, 0xff, 0xdead, 0];
   String decoded = allowInvalidCodec.decode(invalidBytes);
   Expect.equals("\x00\x01\uFFFD\uFFFD\x00", decoded);
   decoded = allowInvalidCodec.decoder.convert(invalidBytes);
   Expect.equals("\x00\x01\uFFFD\uFFFD\x00", decoded);
   decoded = ASCII.decode(invalidBytes, allowInvalid: true);
   Expect.equals("\x00\x01\uFFFD\uFFFD\x00", decoded);
@@ skipping 62 matching lines @@
       }
       for (var nonAsciiString in nonAsciiStrings) {
         var units = nonAsciiString.codeUnits.toList();
         Expect.throws(() {
           decode(units, chunkSize, converter);
         });
       }
     }
   }
 }