Fix unguarded cast to an impl class (issue 26035)

pkg/analyzer/lib/src/dart/element/element.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 analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3574 matching lines @@
     // cycle information for all nodes that are between the target and the
     // node in the topological order.  For simplicity, we simply invalidate
     // all nodes which are reachable from the the source node.
     // Note that in the invalidation phase, we do not cut off when we encounter
     // a node with no library cycle information, since we do not know whether
     // we are in the case where invalidation has already been performed, or we
     // are in the case where library cycles have simply never been computed from
     // a newly reachable node.
     Set<LibraryElementImpl> active = new HashSet();
     void invalidate(LibraryElement library) {
+      if (library is LibraryElementHandle) {
+        library = (library as LibraryElementHandle).actualElement;
+      }
       LibraryElementImpl libraryImpl = library;
       if (active.add(libraryImpl)) {
         if (libraryImpl._libraryCycle != null) {
           libraryImpl._libraryCycle.forEach(invalidate);
           libraryImpl._libraryCycle = null;
         }
         library.exportedLibraries.forEach(invalidate);
         library.importedLibraries.forEach(invalidate);
       }
     }
@@ skipping 1319 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }