We need a CompilationUnit with element.library set.

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

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 6d861b5df45b482ca35294202f4ca16aad840c48..22ac5e57927c2a9572a9f9946c448dbf14a3e678 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -1871,6 +1871,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
           CacheEntry unitEntry =
               getCacheEntry(new LibrarySpecificUnit(librarySource, source));
           CompilationUnit oldUnit = RESOLVED_UNIT_RESULTS
+              .skipWhile((result) => result != RESOLVED_UNIT2)
               .map(unitEntry.getValue)
               .firstWhere((unit) => unit != null, orElse: () => null);
           // If we have the old unit, we can try to update it.