Fix regression in String.toUpperCase becoming locale-dependent

Introduced in 7db3a9b966fac5e397a162c23d68aa220662bec0,
in GWT 2.8.0.

Bug: #9540
Bug-Link: https://github.com/gwtproject/gwt/issues/9540
Change-Id: I7e20b435c63a76934cb8194fd5cd86abfe9b528b
diff --git a/user/super/com/google/gwt/emul/java/lang/String.java b/user/super/com/google/gwt/emul/java/lang/String.java
index aa09004..1833b2f 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -718,7 +718,7 @@
 
   // See the notes in lowerCase pair.
   public String toUpperCase() {
-    return asNativeString().toLocaleUpperCase();
+    return asNativeString().toUpperCase();
   }
 
   // See the notes in lowerCase pair.