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

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 1c45917e8ef2430d05e2090cb2d36114bf6bf85b..2519e41dc04fadd83683a5fe17e56dc2ae62786b 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -825,14 +825,17 @@ class FileSystemState {
 
   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;
+      });
+    }
   }
 }