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

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
@@ skipping 256 matching lines @@
     expect(_excludeSdk(file.importedFiles), isEmpty);
     expect(file.exportedFiles, isEmpty);
     expect(file.partedFiles, isEmpty);
     expect(_excludeSdk(file.directReferencedFiles), isEmpty);
     expect(file.isPart, isFalse);
     expect(file.library, isNull);
     expect(file.unlinked, isNotNull);
     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');
     String a3 = _p('/aaa/lib/a3.dart');
     String a4 = _p('/aaa/lib/a4.dart');
     String b1 = _p('/bbb/lib/b1.dart');
     String b2 = _p('/bbb/lib/b2.dart');
     String content_a1 = r'''
 import 'package:aaa/a2.dart';
 import 'package:bbb/b1.dart';
@@ skipping 432 matching lines @@
   String _p(String path) => provider.convertPath(path);
 
   static String _md5(String content) {
     return hex.encode(md5.convert(UTF8.encode(content)).bytes);
   }
 }
 
 class _GeneratedUriResolverMock extends TypedMock implements UriResolver {}
 
 class _SourceMock extends TypedMock implements Source {}