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

pkg/analyzer_experimental/bin/formatter.dart

 #!/usr/bin/env 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 'dart:io';
 
 import 'package:args/args.dart';
 
@@ skipping 19 matching lines @@
 _formatFiles(files) {
   for (var file in files) {
     _formatFile(file);
   }
 }
 
 _formatFile(path) {
   var buffer = new StringBuffer();
   var file = new File(path);
   file.openRead()
-      .transform(new StringDecoder())
+      .transform(UTF8.decoder)
       .listen((data) =>  buffer.write(data),
         onError: (error) => print('Error, could not open "$path"'),
         onDone: () => print(_formatCU(buffer.toString())));
 }
 
 _formatStdin(options) {
   _log('not supported yet!');
 //  stdin.transform(new StringDecoder())
 //      .listen((String data) => print(data),
 //        onError: (error) => print('Error reading from stdin'),
@@ skipping 33 matching lines @@
 
 /// Format the given [src] as a compilation unit.
 String _formatCU(src, {options: const FormatterOptions()}) =>
     new CodeFormatter(options).format(CodeKind.COMPILATION_UNIT, src);
 
 /// Log the given [msg].
 _log(String msg) {
   //TODO(pquitslund): add proper log support
   print(msg);
 }