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

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 661 matching lines @@
         visitedClasses = <ClassElementImpl>[this];
       } else {
         if (visitedClasses.contains(this)) {
           // Loop in the class hierarchy.  Don't try to forward any
           // constructors.
           return <ConstructorElement>[];
         }
         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();
       }
     }
 
     // Figure out the type parameter substitution we need to perform in order
     // to produce constructors for this class.  We want to be robust in the
     // face of errors, so drop any extra type arguments and fill in any missing
     // ones with `dynamic`.
     List<DartType> parameterTypes =
@@ skipping 4232 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }