[dart:io][windows] Make unicode characters display correctly.

tests/standalone/io/console_unicode_test.dart

+// Copyright (c) 2017, 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:convert';
+import 'dart:io';
+
+main() {
+  String spades = "These are three black spades: ♠♠♠";
+  String german = "German characters: aäbcdefghijklmnoöpqrsßtuüvwxyz";
+
+  stdout.writeln(spades);
+  stdout.writeln(german);
+  print(spades);
+  print(german);
+
+  stdout.add(spades.runes.toList());
+  stdout.writeln();
+  stdout.add(german.runes.toList());
+  stdout.writeln();
+
+  stdout.add(spades.codeUnits);
+  stdout.writeln();
+  stdout.add(german.codeUnits);
+  stdout.writeln();
+
+  stdout.writeln(spades);
+  stdout.writeln(german);
+  print(spades);
+  print(german);
+}