Fix unguarded cast to an impl class (issue 26035)

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 04cdec651d799b4892edc2b5363d8b1390484ac4..d00ab5799dd8c61e492753b216339c22fcb0a550 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -3592,6 +3592,9 @@ class LibraryElementImpl extends ElementImpl implements LibraryElement {
     // 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) {