Add writeAsStringSync (issue 27109)

pkg/analyzer/lib/file_system/physical_file_system.dart

Index: pkg/analyzer/lib/file_system/physical_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/physical_file_system.dart b/pkg/analyzer/lib/file_system/physical_file_system.dart
index 3b02335b012fd745a0c545662432a8dd6c61d10d..9b95ce8d98762285d24a1647c398181560a04f83 100644
--- a/pkg/analyzer/lib/file_system/physical_file_system.dart
+++ b/pkg/analyzer/lib/file_system/physical_file_system.dart
@@ -189,6 +189,16 @@ class _PhysicalFile extends _PhysicalResource implements File {
       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);
+    }
+  }
 }
 
 /**