Fix for subscribing for notifications for files in packages.

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 8af64b3f40d93bef9ec327223660860c200bfc75..5debcd222f1d6ef6e08d3b77c2f3b1a768cc03c1 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -612,6 +612,9 @@ abstract class ContextManager {
     if (context == null) {
       return source;
     }
+    if (context.sourceFactory == null) {
+      return null;
+    }
     Uri uri = context.sourceFactory.restoreUri(source);
     return file.createSource(uri);
   }