Improve the encoding/decoding to/from system encoding on Windows

tests/standalone/io/system_encoding_test.dart

+// Copyright (c) 2015, 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 'dart:io';
+import 'dart:convert';
+
+import "package:expect/expect.dart";
+
+// This only works reliabily for "ASCII" cross platform as that is the only
+// well known part of the default Windows code page.
+void testEncodeDecode(String str) {
+  Expect.equals(
+      SYSTEM_ENCODING.decode(SYSTEM_ENCODING.encode(str)), str);
+}
+
+// This only works reliabily for "ASCII" cross platform as that is the only
+// common set of bytes between UTF-8 Windows code pages that convert back
+// and forth.
+void testDecodeEncode(List<int> bytes) {
+  Expect.listEquals(
+      SYSTEM_ENCODING.encode(SYSTEM_ENCODING.decode(bytes)), bytes);
+}
+
+void test(List<int> bytes) {
+  var str = new String.fromCharCodes(bytes);
+  Expect.equals(SYSTEM_ENCODING.decode(bytes), str);
+  Expect.listEquals(SYSTEM_ENCODING.encode(str), bytes);
+  testDecodeEncode(bytes);
+  testEncodeDecode(str);
+}
+
+main() {
+  test([65, 66, 67]);
+  test([65, 0, 67]);
+  test([0, 65, 0, 67, 0]);
+  test([0, 0, 0]);
+  test(new Iterable.generate(128, (i) => i).toList());
+  // This might break on some Windows systems.
+  testEncodeDecode('\u00c6\u00d8\u00c5');
+  if (Platform.isWindows) {
+    // On Windows the default Windows code page cannot encode these
+    // Unicode characters and the ? character is used.
+    Expect.listEquals(
+        SYSTEM_ENCODING.encode('\u1234\u5678\u9abc'),
+        '???'.codeUnits);
+  } else {
+    // On all systems except for Windows UTF-8 is used as the system
+    // encoding.
+    Expect.listEquals(
+        SYSTEM_ENCODING.encode('\u1234\u5678\u9abc'),
+        UTF8.encode('\u1234\u5678\u9abc'));
+  }
+}