dart2js: Fix off-by-one in assert.

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;
 
 abstract class _MinifiedFieldNamer implements Namer {
   _FieldNamingRegistry get fieldRegistry;
 
   // Returns a minimal name for the field that is globally unique along
@@ skipping 53 matching lines @@
   //
   // The implementation assumes that names are requedsted in order, that is the
   // name at position i+1 is requested after the name at position i was
   // requested.
   jsAst.Name getName(int index) {
     if (index >= nameStore.length) {
       // The namer usually does not use certain names as they clash with
       // existing properties on JS objects (see [_reservedNativeProperties]).
       // However, some of them are really short and safe to use for fields.
       // Thus, we shortcut the namer to use those first.
-      assert(index == nameStore.length + 1);
+      assert(index == nameStore.length);
       if (index < MinifyNamer._reservedNativeProperties.length &&
           MinifyNamer._reservedNativeProperties[index].length <= 2) {
         nameStore.add(
             new StringBackedName(MinifyNamer._reservedNativeProperties[index]));
       } else {
         nameStore.add(namer.getFreshName("field$index", namer.usedInstanceNames,
             namer.suggestedInstanceNames));
       }
     }
 
@@ skipping 144 matching lines @@
       : super.rootScope(box, registry);
 
   @override
   bool containsField(_) => true;
 
   jsAst.Name operator [](Element field) {
     if (!names.containsKey(field)) add(field);
     return names[field];
   }
 }