Treat ambiguous types as malformed.

sdk/lib/_internal/compiler/implementation/js_backend/namer.dart

 // Copyright (c) 2011, 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.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class Namer implements ClosureNamer {
@@ skipping 706 matching lines @@
         // The name is not guaranteed to be unique.
         return safeName(guess);
       }
       if (kind == ElementKind.GENERATIVE_CONSTRUCTOR ||
           kind == ElementKind.FUNCTION ||
           kind == ElementKind.CLASS ||
           kind == ElementKind.FIELD ||
           kind == ElementKind.GETTER ||
           kind == ElementKind.SETTER ||
           kind == ElementKind.TYPEDEF ||
-          kind == ElementKind.LIBRARY ||
-          kind == ElementKind.MALFORMED_TYPE) {
+          kind == ElementKind.LIBRARY) {
         bool fixedName = false;
         if (kind == ElementKind.CLASS) {
           ClassElement classElement = element;
         }
         if (Elements.isInstanceField(element)) {
           fixedName = element.hasFixedBackendName();
         }
         String result = fixedName
             ? guess
             : getFreshName(guess, usedGlobalNames, suggestedGlobalNames,
@@ skipping 524 matching lines @@
         if (!first) {
           sb.write('_');
         }
         sb.write('_');
         visit(link.head);
         first = true;
       }
     }
   }
 }