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

sdk/lib/convert/ascii.dart

Index: sdk/lib/convert/ascii.dart
diff --git a/sdk/lib/convert/ascii.dart b/sdk/lib/convert/ascii.dart
index bea9680aea207064ff8311e34b3e61aaa9365c34..56d6c89b6938523d5865ba00e91cc604331924c9 100644
--- a/sdk/lib/convert/ascii.dart
+++ b/sdk/lib/convert/ascii.dart
@@ -24,7 +24,7 @@ const int _ASCII_MASK = 0x7F;
  * An [AsciiCodec] allows encoding strings as ASCII bytes
  * and decoding ASCII bytes to strings.
  */
-class AsciiCodec extends _Encoding {
+class AsciiCodec extends Encoding {
   final bool _allowInvalid;
   /**
    * Instantiates a new [AsciiCodec].
@@ -39,6 +39,8 @@ class AsciiCodec extends _Encoding {
    */
   const AsciiCodec({bool allowInvalid: false}) : _allowInvalid = allowInvalid;
 
+  String get name => "us-ascii";
+
   /**
    * Decodes the ASCII [bytes] (a list of unsigned 7-bit integers) to the
    * corresponding string.