Remove unnecessary casts from JRE internal

Change-Id: I607e6feee54ea8ad050e5098d872a01bb409a2ed
diff --git a/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java b/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
index 87c404d..b2f128c 100644
--- a/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
+++ b/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
@@ -59,6 +59,7 @@
     return o;
   }-*/;
 
+  @UncheckedCast
   public static native <T> T getProperty(Object map, String key) /*-{
     return map[key];
   }-*/;