Add assertion to UpdateModifiers

Author: rbuckton

spec.html

@@ -37289,6 +37289,7 @@ <h1>
           <dl class="header">
           </dl>
           <emu-alg>
+            1. Assert: _add_ and _remove_ have no elements in common.
             1. Let _ignoreCase_ be _rer_.[[IgnoreCase]].
             1. Let _multiline_ be _rer_.[[Multiline]].
             1. Let _dotAll_ be _rer_.[[DotAll]].