dart2js: Fix off-by-one in assert.

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

Index: pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
diff --git a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
index 61383b92f79061dd0ec85957a729acfad1abe8e6..1666ca52da706d712442797575ec0a188586ee41 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -71,7 +71,7 @@ class _FieldNamingRegistry {
       // 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(