Add UnlinkedUnit.isPartOf flag.

pkg/analyzer/test/src/summary/summary_common.dart

Index: pkg/analyzer/test/src/summary/summary_common.dart
diff --git a/pkg/analyzer/test/src/summary/summary_common.dart b/pkg/analyzer/test/src/summary/summary_common.dart
index a5834acc2bfcb872e256a0f26fad34602fdffa80..527388bb4d78865f4fcddccb61a4a324e5fee715 100644
--- a/pkg/analyzer/test/src/summary/summary_common.dart
+++ b/pkg/analyzer/test/src/summary/summary_common.dart
@@ -9274,6 +9274,13 @@ f(x) => 42;
     expect(unlinkedUnits[1].publicNamespace.names[0].name, 'C');
   }
 
+  test_part_isPartOf() {
+    addNamedSource('/a.dart', 'part of foo; class C {}');
+    serializeLibraryText('library foo; part "a.dart";');
+    expect(unlinkedUnits[0].isPartOf, isFalse);
+    expect(unlinkedUnits[1].isPartOf, isTrue);
+  }
+
   test_reference_zero() {
     // Element zero of the references table should be populated in a standard
     // way.