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

pkg/analysis_server/lib/src/services/search/search_engine.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 services.search_engine;
 
 import 'dart:async';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
@@ skipping 137 matching lines @@
 
   Source _librarySource;
   Source _unitSource;
   LibraryElement _libraryElement;
   Element _element;
 
   SearchMatch(this.context, this.libraryUri, this.unitUri, this.kind,
       this.sourceRange, this.isResolved, this.isQualified);
 
   /**
-   * Return the [Element] containing the match.
+   * Return the [Element] containing the match. Can return `null` if the unit
+   * does not exist, or its element was invalidated, or the element cannot be
+   * found, etc.
    */
   Element get element {
     if (_element == null) {
       CompilationUnitElement unitElement =
           context.getCompilationUnitElement(unitSource, librarySource);
-      _ContainingElementFinder finder =
-          new _ContainingElementFinder(sourceRange.offset);
-      unitElement.accept(finder);
-      _element = finder.containingElement;
+      if (unitElement != null) {
+        _ContainingElementFinder finder =
+            new _ContainingElementFinder(sourceRange.offset);
+        unitElement.accept(finder);
+        _element = finder.containingElement;
+      }
     }
     return _element;
   }
 
   /**
    * The absolute path of the file containing the match.
    */
   String get file => unitSource.fullName;
 
   @override
@@ skipping 52 matching lines @@
     buffer.write(unitUri);
     buffer.write(", range=");
     buffer.write(sourceRange);
     buffer.write(", isResolved=");
     buffer.write(isResolved);
     buffer.write(", isQualified=");
     buffer.write(isQualified);
     buffer.write(")");
     return buffer.toString();
   }
+
+  /**
+   * Return elements of [matches] which has not-null elements.
+   *
+   * When [SearchMatch.element] is not `null` we cache its value, so it cannot
+   * become `null` later.
+   */
+  static List<SearchMatch> withNotNullElement(List<SearchMatch> matches) {
+    return matches.where((match) => match.element != null).toList();
+  }
 }
 
 /**
  * A visitor that finds the deep-most [Element] that contains the [offset].
  */
 class _ContainingElementFinder extends GeneralizingElementVisitor {
   final int offset;
   Element containingElement;
 
   _ContainingElementFinder(this.offset);
 
   visitElement(Element element) {
     if (element is ElementImpl) {
       if (element.codeOffset != null &&
           element.codeOffset <= offset &&
           offset <= element.codeOffset + element.codeLength) {
         containingElement = element;
         super.visitElement(element);
       }
     }
   }
 }