Make sure GWT never emits '$stats' global

That would conflict with lightweight metrics.

Bug: #9196
Bug-Link: https://github.com/gwtproject/gwt/issues/9196
Change-Id: I9970b22e9e038b82f97c6a4fa7c8d0ae3c991e49
diff --git a/dev/core/src/com/google/gwt/dev/js/ast/JsRootScope.java b/dev/core/src/com/google/gwt/dev/js/ast/JsRootScope.java
index f936585..6420f82 100644
--- a/dev/core/src/com/google/gwt/dev/js/ast/JsRootScope.java
+++ b/dev/core/src/com/google/gwt/dev/js/ast/JsRootScope.java
@@ -329,6 +329,7 @@
 
       // GWT-defined identifiers
       "$wnd", "$doc", "$moduleName", "$moduleBase", "$gwt_version", "$sessionId", "gwtOnLoad",
+      "$stats",
       // Any new gwt-related global should be a property of $gwt.
       "$gwt",