Reapply change to use system code page on Windows to fix Pub.

sdk/lib/io/input_stream.dart

Index: sdk/lib/io/input_stream.dart
diff --git a/sdk/lib/io/input_stream.dart b/sdk/lib/io/input_stream.dart
index a59f603f10318d5003f213d6b9a1ce233cb22349..a2e4701ad0aaa5cf0eded1effd4c18ab32f7cfdb 100644
--- a/sdk/lib/io/input_stream.dart
+++ b/sdk/lib/io/input_stream.dart
@@ -103,6 +103,11 @@ class Encoding {
   static const Encoding UTF_8 = const Encoding._internal("UTF-8");
   static const Encoding ISO_8859_1 = const Encoding._internal("ISO-8859-1");
   static const Encoding ASCII = const Encoding._internal("ASCII");
+  /**
+   * SYSTEM encoding is the current code page on Windows and UTF-8 on
+   * Linux and Mac.
+   */
+  static const Encoding SYSTEM = const Encoding._internal("SYSTEM");
   const Encoding._internal(String this.name);
   final String name;
 }