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

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

 // Copyright (c) 2014, 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 test.services.src.search.search_engine_internal;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
@@ skipping 599 matching lines @@
 }
 ''');
     MethodMember method = findNodeElementAtString('m(); // ref');
     Element mainElement = findElement('main');
     var expected = [
       _expectIdQ(mainElement, MatchKind.INVOCATION, 'm(); // ref')
     ];
     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 {
   var f;
   C({p}) : f = p + 1 {
     p = 2;
     p += 3;
     print(p);
     p();
   }
@@ skipping 298 matching lines @@
     List<SearchMatch> matches = await searchEngine.searchReferences(element);
     _assertMatches(matches, expectedMatches);
     expect(matches, hasLength(expectedMatches.length));
   }
 
   static void _assertMatches(
       List<SearchMatch> matches, List<ExpectedMatch> expectedMatches) {
     expect(matches, unorderedEquals(expectedMatches));
   }
 }