Adds @SuppressWarnings("unusable-by-js") for HasJavaThrowable

Change-Id: I03d5f95e8f3950b88fefdeb33aad6892abbdc9b4
diff --git a/user/super/com/google/gwt/emul/java/lang/Throwable.java b/user/super/com/google/gwt/emul/java/lang/Throwable.java
index ae36177..3eaa58a 100644
--- a/user/super/com/google/gwt/emul/java/lang/Throwable.java
+++ b/user/super/com/google/gwt/emul/java/lang/Throwable.java
@@ -315,6 +315,7 @@
   @JsType(isNative = true, name = "TypeError", namespace = "<window>")
   private static class NativeTypeError { }
 
+  @SuppressWarnings("unusable-by-js")
   @JsType(isNative = true, name = "?", namespace = JsPackage.GLOBAL)
   private interface HasJavaThrowable {
     @JsProperty(name = "__java$exception")