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

pkg/analyzer/lib/src/dart/analysis/file_state.dart

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 619a0350b48fe0d307c4d2003a48e2d1b197dc47..facf11f3a5912bfeab5009df057429bfb36ab812 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -191,9 +191,10 @@ class FileState {
   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