Improve streaming UTF-8 decoder

tests/standalone/io/string_decoder_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.
+
+part '../../../sdk/lib/io/buffer_list.dart';
+part '../../../sdk/lib/io/input_stream.dart';
+part '../../../sdk/lib/io/string_stream.dart';
+
+void test() {
+  // Code point U+10FFFF is the largest code point supported by Dart.
+  var decoder = _StringDecoders.decoder(Encoding.UTF_8);
+  decoder.write([0xf0, 0x90, 0x80, 0x80]);  // U+10000
+  decoder.write([0xf4, 0x8f, 0xbf, 0xbf]);  // U+10FFFF
+  decoder.write([0xf4, 0x90, 0x80, 0x80]);  // U+110000
+  decoder.write([0xfa, 0x80, 0x80, 0x80, 0x80]);  //  U+2000000
+  decoder.write([0xfd, 0x80, 0x80, 0x80, 0x80, 0x80]);  // U+40000000
+
+  var decoded = decoder.decoded();
+  Expect.equals(7, decoded.length);
+
+  var replacementChar = '?'.charCodeAt(0);
+  Expect.equals(0xd800, decoded.charCodeAt(0));
+  Expect.equals(0xdc00, decoded.charCodeAt(1));
+  Expect.equals(0xdbff, decoded.charCodeAt(2));
+  Expect.equals(0xdfff, decoded.charCodeAt(3));
+  Expect.equals(replacementChar, decoded.charCodeAt(4));
+  Expect.equals(replacementChar, decoded.charCodeAt(5));
+  Expect.equals(replacementChar, decoded.charCodeAt(6));
+}
+
+void testInvalid() {
+  var decoder = _StringDecoders.decoder(Encoding.UTF_8);
+  Expect.throws(() => decoder.write([0x80]));
+  Expect.throws(() => decoder.write([0xff]));
+  Expect.throws(() => decoder.write([0xf0, 0xc0]));
+}
+
+void main() {
+  test();
+  testInvalid();
+}