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

tests/lib/convert/utf8_encode_test.dart

Index: tests/lib/convert/utf8_encode_test.dart
diff --git a/tests/lib/convert/utf8_encode_test.dart b/tests/lib/convert/utf8_encode_test.dart
index 4e15646347a6e4708992a2fdadede1d2867b1c66..489a12460e48c4fcbf3cec6df05a346e9c2d636c 100644
--- a/tests/lib/convert/utf8_encode_test.dart
+++ b/tests/lib/convert/utf8_encode_test.dart
@@ -9,11 +9,54 @@ import 'unicode_tests.dart';
 List<int> encode(String str) => new Utf8Encoder().convert(str);
 List<int> encode2(String str) => UTF8.encode(str);
 
-main() {
+void main() {
   for (var test in UNICODE_TESTS) {
     List<int> bytes = test[0];
     String string = test[1];
     Expect.listEquals(bytes, encode(string));
     Expect.listEquals(bytes, encode2(string));
   }
+
+  testEncodeSlice();
+}
+
+void testEncodeSlice() {
+  var encoder = UTF8.encoder;
+  String ascii = "ABCDE";
+  Expect.listEquals([0x41, 0x42, 0x43, 0x44, 0x45],
+                    encoder.convert(ascii));
+  Expect.listEquals([0x41, 0x42, 0x43, 0x44, 0x45],
+                    encoder.convert(ascii, 0));
+  Expect.listEquals([0x41, 0x42, 0x43, 0x44, 0x45],
+                    encoder.convert(ascii, 0, 5));
+  Expect.listEquals([0x42, 0x43, 0x44, 0x45],
+                    encoder.convert(ascii, 1));
+  Expect.listEquals([0x41, 0x42, 0x43, 0x44],
+                    encoder.convert(ascii, 0, 4));
+  Expect.listEquals([0x42, 0x43, 0x44],
+                    encoder.convert(ascii, 1, 4));
+
+  Expect.throws(() => encoder.convert(ascii, -1));    // start < 0.
+  Expect.throws(() => encoder.convert(ascii, 6));     // start > length
+  Expect.throws(() => encoder.convert(ascii, 0, -1)); // end < 0
+  Expect.throws(() => encoder.convert(ascii, 0, 6));  // end > length
+  Expect.throws(() => encoder.convert(ascii, 3, 2));  // end < start
+
+  var unicode = "\u0081\u0082\u1041\u{10101}";
+
+  Expect.listEquals([0xc2, 0x81, 0xc2, 0x82, 0xe1, 0x81, 0x81,
+                     0xf0, 0x90, 0x84, 0x81],
+                    encoder.convert(unicode));
+  Expect.listEquals([0xc2, 0x81, 0xc2, 0x82, 0xe1, 0x81, 0x81,
+                     0xf0, 0x90, 0x84, 0x81],
+                    encoder.convert(unicode, 0, unicode.length));
+  Expect.listEquals([0xc2, 0x82, 0xe1, 0x81, 0x81,
+                     0xf0, 0x90, 0x84, 0x81],
+                    encoder.convert(unicode, 1));
+  Expect.listEquals([0xc2, 0x82, 0xe1, 0x81, 0x81],
+                    encoder.convert(unicode, 1, 3));
+  // Split in the middle of a surrogate pair.
+  Expect.listEquals([0xc2, 0x82, 0xe1, 0x81, 0x81,
+                     0xed, 0xa0, 0x80],
+                    encoder.convert(unicode, 1, 4));
 }