Remove Encoding-enum from dart:io and add interface in dart:convert.

sdk/lib/io/stdio.dart

Index: sdk/lib/io/stdio.dart
diff --git a/sdk/lib/io/stdio.dart b/sdk/lib/io/stdio.dart
index 3eaa9697a6757392f09be80c0ececf2ae44bbe6f..372552624bc00a8f78c6a90ccbd646ca7fe61cc9 100644
--- a/sdk/lib/io/stdio.dart
+++ b/sdk/lib/io/stdio.dart
@@ -58,7 +58,7 @@ class Stdin extends _StdStream implements Stream<List<int>> {
    * If end-of-file is reached after some data has already been read, that data
    * is returned.
    */
-  String readLineSync({Encoding encoding: Encoding.SYSTEM,
+  String readLineSync({Encoding encoding: SYSTEM_ENCODING,
                        bool retainNewlines: false}) {
     const CR = 13;
     const LF = 10;
@@ -69,7 +69,7 @@ class Stdin extends _StdStream implements Stream<List<int>> {
 
     StreamController<List<int>> controller =
         new StreamController<List<int>>(sync: true);
-    Stream stream = controller.stream.transform(new StringDecoder(encoding));
+    Stream stream = controller.stream.transform(encoding.decoder);
     stream.listen((String str) {
       line.write(str);
     }, onError: (e) {