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

tests/lib/convert/utf8_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';
 import 'unicode_tests.dart';
 
 String decode(List<int> bytes) => new Utf8Decoder().convert(bytes);
 
-main() {
+void main() {
   for (var test in UNICODE_TESTS) {
     List<int> bytes = test[0];
     String expected = test[1];
     Expect.stringEquals(expected, decode(bytes));
   }
+
+  testDecodeSlice();
 }
+
+void testDecodeSlice() {
+  var decoder = UTF8.decoder;  // Doesn't allow malformed.
+  var ascii = [0x41, 0x42, 0x43, 0x44, 0x45];
+  Expect.equals("ABCDE", decoder.convert(ascii));
+  Expect.equals("ABCDE", decoder.convert(ascii, 0));
+  Expect.equals("ABCDE", decoder.convert(ascii, 0, ascii.length));
+  Expect.equals("CDE", decoder.convert(ascii, 2));
+  Expect.equals("BCD", decoder.convert(ascii, 1, 4));
+  Expect.equals("ABCD", decoder.convert(ascii, 0, 4));
+
+  Expect.throws(() => decoder.convert(ascii, -1));    // start < 0.
+  Expect.throws(() => decoder.convert(ascii, 6));     // start > length
+  Expect.throws(() => decoder.convert(ascii, 0, -1)); // end < 0
+  Expect.throws(() => decoder.convert(ascii, 0, 6));  // end > length
+  Expect.throws(() => decoder.convert(ascii, 3, 2));  // end < start
+
+  var utf8 = [0xc2, 0x81, 0xc2, 0x82, 0xe1, 0x81, 0x81];
+  Expect.equals("\u0081\u0082\u1041", decoder.convert(utf8));
+  Expect.equals("\u0082\u1041", decoder.convert(utf8, 2));
+  Expect.equals("\u0081\u0082", decoder.convert(utf8, 0, 4));
+  Expect.equals("\u0082", decoder.convert(utf8, 2, 4));
+  Expect.throws(() => decoder.convert(utf8, 1));
+  Expect.throws(() => decoder.convert(utf8, 0, 1));
+  Expect.throws(() => decoder.convert(utf8, 2, 5));
+}