Issue 30006. Don't flush on write to FileByteStore.

pkg/front_end/lib/src/incremental/file_byte_store.dart

 // Copyright (c) 2015, 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:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import 'package:front_end/src/incremental/byte_store.dart';
 import 'package:path/path.dart';
@@ skipping 141 matching lines @@
       return _getFileForKey(key).readAsBytesSync();
     } catch (_) {
       return null;
     }
   }
 
   @override
   void put(String key, List<int> bytes) {
     try {
       File tempFile = _getFileForKey(_tempName);
-      tempFile.writeAsBytesSync(bytes, flush: true);
+      tempFile.writeAsBytesSync(bytes);
       File file = _getFileForKey(key);
       tempFile.renameSync(file.path);
     } catch (_) {}
   }
 
   File _getFileForKey(String key) {
     return new File(join(_cachePath, key));
   }
 }