Consider MultiplyDefinedElement as null for summary serialization.

Bug: https://buganizer.corp.google.com/issues/142145283
Change-Id: I4de873d322236d97fb942dbb0ae3f4fc22a0687e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120440
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/summary2/linking_bundle_context.dart

@@ -51,6 +51,7 @@ class LinkingBundleContext {
 
   int indexOfElement(Element element) {
     if (element == null) return 0;
+    if (element is MultiplyDefinedElement) return 0;
     assert(element is! Member);
 
     if (identical(element, DynamicElementImpl.instance)) {

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

@@ -10867,12 +10867,48 @@ class C {
 
   test_unresolved_annotation_simpleIdentifier() async {
     var library = await checkLibrary('@foo class C {}', allowErrors: true);
-    checkElementText(library, r'''
-@
-        foo/*location: null*/
+    checkElementText(
+        library,
+        r'''
 class C {
 }
+  metadata
+    Annotation
+      element: <null>
+      name: SimpleIdentifier
+        staticElement: <null>
+        staticType: dynamic
+        token: foo
+''',
+        withFullyResolvedAst: true);
+  }
+
+  test_unresolved_annotation_simpleIdentifier_multiplyDefined() async {
+    addLibrarySource('/a.dart', 'const v = 0;');
+    addLibrarySource('/b.dart', 'const v = 0;');
+    var library = await checkLibrary('''
+import 'a.dart';
+import 'b.dart';
+
+@v
+class C {}
 ''');
+    checkElementText(
+        library,
+        r'''
+import 'a.dart';
+import 'b.dart';
+class C {
+}
+  metadata
+    Annotation
+      element: <null>
+      name: SimpleIdentifier
+        staticElement: <null>
+        staticType: dynamic
+        token: v
+''',
+        withFullyResolvedAst: true);
   }
 
   test_unresolved_annotation_unnamedConstructorCall_noClass() async {