Cover some more cases.

lib/compiler/implementation/unparse_validator.dart

 // Copyright (c) 2012, 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.
 
 class ValidatorListener implements DiagnosticListener {
   final SourceFile sourceFile;
 
   ValidatorListener(this.sourceFile);
 
   void cancel([String reason, node, token, instruction, element]) {
@@ skipping 53 matching lines @@
 
   void check(Element element) {
     if (!validateUnparse) return;
 
     if (element is PartialFunctionElement) {
       checkFunction(element);
     } else if (element.isGenerativeConstructor()) {
       assert(element is FunctionElement);
       // Generative constructors parse to very special function expressions.
       // Handle them when classes are properly handled.
-    } else if (element.isField()) {
-      assert(element is VariableElement);
+    } else if (element.isField() || element is AbstractFieldElement) {
+      assert(element is VariableElement || element is AbstractFieldElement);
       // Fields are just names with possible initialization expressions.
       // Nothing to care about for now.
+    } else if (element is VoidElement) {
+      // Nothing to do here.
     } else {
-      compiler.cancel('Cannot handle $element', element: element);
+      compiler.cancel('Cannot handle $element');
     }
   }
 }