Make a copy of files to invalidate to avoid concurrent modifications.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

Index: pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
diff --git a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
index 499e3fa70f8921c770e653756b2e672e57925aa4..3a23501b1d694ef00a532fa51be769e0dadaf677 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -282,14 +282,18 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
   /// Refresh all the invalidated files and update dependencies.
   Future<Null> _refreshInvalidatedFiles() async {
     await _logger.runAsync('Refresh invalidated files', () async {
-      for (var fileUri in _invalidatedFiles) {
+      // Create a copy to avoid concurrent modifications.
+      var invalidatedFiles = _invalidatedFiles.toList();
+      _invalidatedFiles.clear();
+
+      // Refresh the files.
+      for (var fileUri in invalidatedFiles) {
         var file = _fsState.getFileByFileUri(fileUri);
         if (file != null) {
           _logger.writeln('Refresh $fileUri');
           await file.refresh();
         }
       }
-      _invalidatedFiles.clear();
     });
   }