Updated reference to missing UTF8 codec.

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:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
 
 import 'package:analyzer_experimental/src/services/formatter_impl.dart';
 
 
 const BINARY_NAME = 'dartfmt';
 final argParser = _initArgParser();
 
@@ skipping 67 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);
 }