Only units with 'part of' and no other directives are parts.

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 8809da3d8e0960276f1aadbe7917453ee5de3912..9aaa8c8c98be79290fedfa40184139109bdc7b5f 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -2681,16 +2681,8 @@ class ParseDartTask extends SourceBasedAnalysisTask {
     SourceKind sourceKind = SourceKind.LIBRARY;
     if (modificationTime == -1) {
       sourceKind = SourceKind.UNKNOWN;
-    } else if (hasNonPartOfDirective) {
-      sourceKind = SourceKind.LIBRARY;
-    } else if (hasPartOfDirective) {
+    } else if (hasPartOfDirective && !hasNonPartOfDirective) {
       sourceKind = SourceKind.PART;
-    } else {
-      if (context.getLibrariesContaining(source).isEmpty) {
-        sourceKind = SourceKind.LIBRARY;
-      } else {
-        sourceKind = SourceKind.PART;
-      }
     }
     //
     // Record outputs.