Fix front-end tests: the original code is correct, this is a workaround for issue #29393

pkg/analyzer/lib/src/dart/analysis/file_state.dart

 // Copyright (c) 2016, 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:convert';
 import 'dart:typed_data';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 807 matching lines @@
       _pathToFiles[path] = files;
       // Schedule the stream update.
       _addedKnownFiles.add(path);
       _scheduleKnownFilesSetChange();
     }
     files.add(file);
   }
 
   void _scheduleKnownFilesSetChange() {
     Duration delay = _knownFilesSetChangesDelay ?? new Duration(seconds: 1);
-    _knownFilesSetChangesTimer ??= new Timer(delay, () {
-      Set<String> addedFiles = _addedKnownFiles.toSet();
-      Set<String> removedFiles = new Set<String>();
-      _knownFilesSetChangesController
-          .add(new KnownFilesSetChange(addedFiles, removedFiles));
-      _addedKnownFiles.clear();
-      _knownFilesSetChangesTimer = null;
-    });
+    // Note: we don't use ??= here due to issue #29393
+    if (_knownFilesSetChangesTimer == null) {
+      _knownFilesSetChangesTimer = new Timer(delay, () {
+        Set<String> addedFiles = _addedKnownFiles.toSet();
+        Set<String> removedFiles = new Set<String>();
+        _knownFilesSetChangesController
+            .add(new KnownFilesSetChange(addedFiles, removedFiles));
+        _addedKnownFiles.clear();
+        _knownFilesSetChangesTimer = null;
+      });
+    }
   }
 }
 
 @visibleForTesting
 class FileSystemStateTestView {
   final FileSystemState state;
 
   FileSystemStateTestView(this.state);
 
   Set<FileState> get filesWithoutTransitiveFiles {
@@ skipping 37 matching lines @@
   _FastaElementProxy operator [](fasta.Builder builder) =>
       _elements.putIfAbsent(builder, () => new _FastaElementProxy());
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
 class _FastaInterfaceTypeProxy implements fasta.KernelInterfaceType {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }