Keep cache entries for explicitly added sources.

pkg/analyzer/lib/src/context/cache.dart

Index: pkg/analyzer/lib/src/context/cache.dart
diff --git a/pkg/analyzer/lib/src/context/cache.dart b/pkg/analyzer/lib/src/context/cache.dart
index 4535ca4ce1d4cea4079b9eafa45152bce0f267f5..4443dfaad38d423c09730c98fc92551fe97e4f19 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -565,8 +565,8 @@ class CacheEntry {
     }
     // Invalidate results that depend on this result.
     _invalidateDependentResults(id, thisData, delta, level + 1);
-    // If empty, remove the entry altogether.
-    if (_resultMap.isEmpty) {
+    // If empty and not explicitly added, remove the entry altogether.
+    if (_resultMap.isEmpty && !explicitlyAdded) {
       _partition._targetMap.remove(target);
       _partition._removeIfSource(target);
     }