Add writeAsStringSync (issue 27109)

pkg/analyzer/lib/file_system/file_system.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.file_system.file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/absolute_path.dart';
@@ skipping 41 matching lines @@
    * an exception is thrown.
    */
   File renameSync(String newPath);
 
   /**
    * Synchronously write a list of bytes to the file.
    *
    * Throws a [FileSystemException] if the operation fails.
    */
   void writeAsBytesSync(List<int> bytes);
+
+  /**
+   * Synchronously write a the given [content] to the file. The new content will
+   * replace any existing content.
+   *
+   * Throws a [FileSystemException] if the operation fails.
+   */
+  void writeAsStringSync(String content);
 }
 
 /**
  * Base class for all file system exceptions.
  */
 class FileSystemException implements Exception {
   final String path;
   final String message;
 
   FileSystemException(this.path, this.message);
@@ skipping 183 matching lines @@
 
   @override
   Uri restoreAbsolute(Source source) =>
       _provider.pathContext.toUri(source.fullName);
 
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }