Fix another unguarded cast to an impl class (issue 26034, case 2)

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 d00ab5799dd8c61e492753b216339c22fcb0a550..c91582c6790c254570da2683ba442e3a558513fd 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -679,9 +679,12 @@ class ClassElementImpl extends ElementImpl implements ClassElement {
         visitedClasses.add(this);
       }
       try {
-        ClassElementImpl superclass = supertype.element;
-        constructorsToForward =
-            superclass._computeMixinAppConstructors(visitedClasses);
+        ClassElement superclass = supertype.element;
+        if (superclass is ClassElementHandle) {
+          superclass = (superclass as ClassElementHandle).actualElement;
+        }
+        constructorsToForward = (superclass as ClassElementImpl)
+            ._computeMixinAppConstructors(visitedClasses);
       } finally {
         visitedClasses.removeLast();
       }