Use failedAt in more places

pkg/compiler/lib/src/js_backend/field_naming_mixin.dart

 // Copyright (c) 2015, 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.namer;
 
 abstract class _MinifiedFieldNamer implements Namer {
   _FieldNamingRegistry get fieldRegistry;
 
   // Returns a minimal name for the field that is globally unique along
@@ skipping 158 matching lines @@
   jsAst.Name operator [](Element field) {
     jsAst.Name name = names[field];
     if (name == null && superScope != null) return superScope[field];
     return name;
   }
 
   void add(Element field) {
     if (names.containsKey(field)) return;
 
     jsAst.Name value = _nextName();
-    assert(invariant(field, _isNameUnused(value)));
+    assert(_isNameUnused(value), failedAt(field));
     names[field] = value;
   }
 
   bool containsField(Element field) => names.containsKey(field);
 }
 
 /**
  * Field names for mixins have two constraints: They need to be unique in the
  * hierarchy of each application of a mixin and they need to be the same for
  * all applications of a mixin. To achieve this, we use global naming for
@@ skipping 35 matching lines @@
       : super.rootScope(box, registry);
 
   @override
   bool containsField(_) => true;
 
   jsAst.Name operator [](Element field) {
     if (!names.containsKey(field)) add(field);
     return names[field];
   }
 }