The file is a library, if it has a 'library' directive.

pkg/analyzer/test/src/dart/analysis/file_state_test.dart

Index: pkg/analyzer/test/src/dart/analysis/file_state_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/file_state_test.dart b/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
index 902eb51429757b2f591fc35a5b3be86e4bfd37e1..7b0157e37a9902dcf6496562a98747318cd64b8e 100644
--- a/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/file_state_test.dart
@@ -274,6 +274,18 @@ class A2 {}
     expect(file.unlinked.classes, isEmpty);
   }
 
+  test_getFileForPath_hasLibraryDirective_hasPartOfDirective() {
+    String a = _p('/test/lib/a.dart');
+    provider.newFile(
+        a,
+        r'''
+library L;
+part of L;
+''');
+    FileState file = fileSystemState.getFileForPath(a);
+    expect(file.isPart, isFalse);
+  }
+
   test_getFileForPath_library() {
     String a1 = _p('/aaa/lib/a1.dart');
     String a2 = _p('/aaa/lib/a2.dart');