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

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

 // Copyright (c) 2015, 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.
 
 library analyzer.src.task.dart;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 2663 matching lines @@
     HashSet<Source> importedSourceSet =
         new HashSet.from(explicitlyImportedSourceSet);
     Source coreLibrarySource = context.sourceFactory.forUri(DartSdk.DART_CORE);
     importedSourceSet.add(coreLibrarySource);
     //
     // Compute kind.
     //
     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.
     //
     List<Source> explicitlyImportedSources =
         explicitlyImportedSourceSet.toList();
     List<Source> exportedSources = exportedSourceSet.toList();
     List<Source> importedSources = importedSourceSet.toList();
     List<Source> includedSources = includedSourceSet.toList();
     List<AnalysisError> parseErrors = errorListener.errors;
@@ skipping 688 matching lines @@
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.first;
     _newSources.remove(currentTarget);
     return true;
   }
 }