Issue 23027. Try to make priority files analyzable.

pkg/analyzer/lib/src/generated/engine.dart

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index 4ad8d0117230c61aa8738e7d60ada590934a264d..d711bc680b6760d4a0ace946a4fcb38b2e6d0caa 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -1105,6 +1105,13 @@ class AnalysisContextImpl implements InternalAnalysisContext {
       for (int i = 0; i < count; i++) {
         _priorityOrder[i] = sources[i];
       }
+      // Ensure entries for every priority source.
+      for (var source in _priorityOrder) {
+        SourceEntry entry = _getReadableSourceEntry(source);
+        if (entry == null) {
+          _createSourceEntry(source, false);
+        }
+      }
     }
   }