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

pkg/analyzer/lib/src/dart/analysis/file_state.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/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 173 matching lines @@
 
   @override
   int get hashCode => uri.hashCode;
 
   /**
    * The list of files this file imports.
    */
   List<FileState> get importedFiles => _importedFiles;
 
   /**
-   * Return `true` if the file has a `part of` directive, so is probably a part.
+   * Return `true` if the file does not have a `library` directive, and has a
+   * `part of` directive, so is probably a part.
    */
-  bool get isPart => _unlinked.isPartOf;
+  bool get isPart => _unlinked.libraryNameOffset == 0 &&  _unlinked.isPartOf;

[Brian Wilkerson, 2017/01/31 20:33:00]

I'm concerned about this because it appears that unlinked summaries have a
different definition of when they are a part than other places in the code base.
I think summaries should be consistent. Could we change `isPartOf` to return
false if there is a library directive?

 
   /**
    * If the file [isPart], return a currently know library the file is a part
    * of. Return `null` if a library is not known, for example because we have
    * not processed a library file yet.
    */
   FileState get library {
     List<FileState> libraries = _fsState._partToLibraries[this];
     if (libraries == null || libraries.isEmpty) {
       return null;
@@ skipping 497 matching lines @@
         .where((f) => f._transitiveFiles == null)
         .toSet();
   }
 
   Set<FileState> get filesWithoutTransitiveSignature {
     return state._uriToFile.values
         .where((f) => f._transitiveSignature == null)
         .toSet();
   }
 }