Issue 27542. Guard against CompilationUnitElement is null in SearchMatch.element getter.

pkg/analysis_server/test/services/search/search_engine_test.dart

Index: pkg/analysis_server/test/services/search/search_engine_test.dart
diff --git a/pkg/analysis_server/test/services/search/search_engine_test.dart b/pkg/analysis_server/test/services/search/search_engine_test.dart
index 86f1d6f0241f6aac737c2931c0228ce1758aa448..8072301d51afba0ca33948440d62f83583c7758b 100644
--- a/pkg/analysis_server/test/services/search/search_engine_test.dart
+++ b/pkg/analysis_server/test/services/search/search_engine_test.dart
@@ -617,6 +617,23 @@ main(A<int> a) {
     await _verifyReferences(method, expected);
   }
 
+  test_searchReferences_null_noUnitElement() async {
+    _indexTestUnit('''
+class A {
+  m() {}
+}
+main(A a) {
+  a.m();
+}
+''');
+    MethodElement method = findElement('m');
+    List<SearchMatch> matches = await searchEngine.searchReferences(method);
+    expect(matches, hasLength(1));
+    // Set the source contents, so the element is invalidated.
+    context.setContents(testSource, '');
+    expect(matches.single.element, isNull);
+  }
+
   test_searchReferences_ParameterElement_ofConstructor() async {
     _indexTestUnit('''
 class C {