Add writeAsStringSync (issue 27109)

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 41d6f848f39f8e45a0d478f10737eba0f14571a7..c02c90ab1c6fa108bc1b30ce1d78a8afcaf31530 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -59,6 +59,14 @@ abstract class File implements Resource {
    * 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);
 }
 
 /**