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

tests/standalone/io/stdout_stderr_test.dart

 // Copyright (c) 2013, 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 "package:expect/expect.dart";
 import "dart:async";
+import "dart:convert";
 import "dart:io";
 
 callIOSink(IOSink sink) {
   // Call all methods on IOSink.
-  sink.encoding = Encoding.ASCII;
-  Expect.equals(Encoding.ASCII, sink.encoding);
+  sink.encoding = ASCII;
+  Expect.equals(ASCII, sink.encoding);
   sink.write("Hello\n");
   sink.writeln("Hello");
   sink.writeAll(["H", "e", "l", "lo\n"]);
   sink.writeCharCode(72);
   sink.add([101, 108, 108, 111, 10]);
 
   var controller = new StreamController(sync: true);
   var future = sink.addStream(controller.stream);
   controller.add([72, 101, 108]);
   controller.add([108, 111, 10]);
@@ skipping 11 matching lines @@
 main() {
   callIOSink(stdout);
   stdout.done.then((_) {
     callIOSink(stderr);
     stderr.done.then((_) {
       stdout.close();
       stderr.close();
     });
   });
 }