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

tests/standalone/io/stdin_sync_test.dart

Index: tests/standalone/io/stdin_sync_test.dart
diff --git a/tests/standalone/io/stdin_sync_test.dart b/tests/standalone/io/stdin_sync_test.dart
index 0ac4e9df5d502d7b374a01244285d53bbc58fa4e..c0477c01ffe781d7fdd9b39145f870188feecce6 100644
--- a/tests/standalone/io/stdin_sync_test.dart
+++ b/tests/standalone/io/stdin_sync_test.dart
@@ -17,14 +17,14 @@ void testReadByte() {
       process.stdin.write(line);
       process.stdin.close();
       process.stderr
-          .transform(new StringDecoder())
+          .transform(UTF8.decoder)
           .transform(new LineSplitter())
           .fold(new StringBuffer(), (b, d) => b..write(d))
           .then((data) {
             if (data.toString() != '') throw "Bad output: '$data'";
           });
       process.stdout
-          .transform(new StringDecoder())
+          .transform(UTF8.decoder)
           .transform(new LineSplitter())
           .fold(new StringBuffer(), (b, d) => b..write(d))
           .then((data) {