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

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

Index: pkg/analyzer/lib/src/task/dart_work_manager.dart
diff --git a/pkg/analyzer/lib/src/task/dart_work_manager.dart b/pkg/analyzer/lib/src/task/dart_work_manager.dart
index e988f1c8e0aafac7b01a13259e1430526642d276..6df4c7e68ef7a15ca3213da1d03b320e6c31583d 100644
--- a/pkg/analyzer/lib/src/task/dart_work_manager.dart
+++ b/pkg/analyzer/lib/src/task/dart_work_manager.dart
@@ -263,25 +263,12 @@ class DartWorkManager implements WorkManager {
       List<Source> includedParts = outputs[INCLUDED_PARTS];
       if (includedParts != null) {
         libraryPartsMap[library] = includedParts;
-        // update contanining libraries
         for (Source part in includedParts) {
           List<Source> libraries =
               partLibrariesMap.putIfAbsent(part, () => <Source>[]);
           if (!libraries.contains(library)) {
             libraries.add(library);
-          }
-        }
-        // all of the "includedParts" are not libraries anymore
-        for (Source part in includedParts) {
-          unknownSourceQueue.remove(part);
-          librarySourceQueue.remove(part);
-          analysisCache.remove(new LibrarySpecificUnit(part, part));
-          CacheEntry partEntry = analysisCache.get(part);
-          if (partEntry != null) {
-            partEntry.setValue(SOURCE_KIND, SourceKind.PART, <TargetedResult>[
-              new TargetedResult(part, CONTENT),
-              new TargetedResult(target, CONTENT)
-            ]);
+            _invalidateContainingLibraries(part);
           }
         }
       }
@@ -324,6 +311,19 @@ class DartWorkManager implements WorkManager {
   }
 
   /**
+   * Invalidate  [CONTAINING_LIBRARIES] for the given [source].
+   * [CONTAINING_LIBRARIES] does not have dependencies, so we manage it here.
+   * The [source] may be a part, or a library whose contents is updated so
+   * will be a part.
+   */
+  void _invalidateContainingLibraries(Source source) {
+    CacheEntry entry = analysisCache.get(source);
+    if (entry != null) {
+      entry.setState(CONTAINING_LIBRARIES, CacheState.INVALID);
+    }
+  }
+
+  /**
    * Returns `true` if the given [result] of the given [target] needs
    * computing, i.e. it is not in the valid and not in the error state.
    */
@@ -343,9 +343,11 @@ class DartWorkManager implements WorkManager {
         List<Source> libraries = partLibrariesMap[part];
         if (libraries != null) {
           libraries.remove(library);
+          _invalidateContainingLibraries(part);

[Brian Wilkerson, 2015/06/04 20:45:56]

I think this does too much. If a file 'part.dart' is included (via part
directive) in both 'libA.dart' and 'libB.dart', and 'libA.dart' is modified,
won't we loose the fact that 'part.dart' is in 'libB.dart'?

[scheglov, 2015/06/04 20:50:27]

Yes, we will lose it in the cache.
So, ContainingLibrariesTask will be run, call context.getLibrariesContaining()
and get this information again into cache by asking DartWorkManager.

We don't nuke this information in DartWorkManager, so "part.dart" will remember
that it is a part of "libB.dart".

         }
       }
     }
+    _invalidateContainingLibraries(library);
   }
 
   static bool _isDartSource(AnalysisTarget target) {