Add writeAsStringSync (issue 27109)

pkg/analyzer/lib/file_system/physical_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.physical_file_system;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
@@ skipping 171 matching lines @@
 
   @override
   void writeAsBytesSync(List<int> bytes) {
     try {
       io.File file = _entry as io.File;
       file.writeAsBytesSync(bytes);
     } on io.FileSystemException catch (exception) {
       throw new FileSystemException(exception.path, exception.message);
     }
   }
+
+  @override
+  void writeAsStringSync(String content) {
+    try {
+      io.File file = _entry as io.File;
+      file.writeAsStringSync(content);
+    } on io.FileSystemException catch (exception) {
+      throw new FileSystemException(exception.path, exception.message);
+    }
+  }
 }
 
 /**
  * A `dart:io` based implementation of [Folder].
  */
 class _PhysicalFolder extends _PhysicalResource implements Folder {
   _PhysicalFolder(io.Directory directory) : super(directory);
 
   @override
   Stream<WatchEvent> get changes => new DirectoryWatcher(_entry.path).events;
@@ skipping 141 matching lines @@
           timer.cancel();
         }
       });
       return completer.future;
     }
     _isSpawning = true;
     _runner = await IsolateRunner.spawn();
     return _runner;
   }
 }