Normative: allow host exotic objects to reject private fields (#2807)

spec.html

@@ -6816,6 +6816,8 @@ <h1>
       <dl class="header">
       </dl>
       <emu-alg>
+        1. If the host is a web browser, then
+          1. Perform ? HostEnsureCanAddPrivateElement(_O_).
         1. Let _entry_ be PrivateElementFind(_O_, _P_).
         1. If _entry_ is not ~empty~, throw a *TypeError* exception.
         1. Append PrivateElement { [[Key]]: _P_, [[Kind]]: ~field~, [[Value]]: _value_ } to _O_.[[PrivateElements]].
@@ -6834,6 +6836,8 @@ <h1>
       </dl>
       <emu-alg>
         1. Assert: _method_.[[Kind]] is either ~method~ or ~accessor~.
+        1. If the host is a web browser, then
+          1. Perform ? HostEnsureCanAddPrivateElement(_O_).
         1. Let _entry_ be PrivateElementFind(_O_, _method_.[[Key]]).
         1. If _entry_ is not ~empty~, throw a *TypeError* exception.
         1. Append _method_ to _O_.[[PrivateElements]].
@@ -6844,6 +6848,25 @@ <h1>
       </emu-note>
     </emu-clause>
 
+    <emu-clause id="sec-hostensurecanaddprivateelement" type="host-defined abstract operation">
+      <h1>
+        HostEnsureCanAddPrivateElement (
+          _O_: an Object,
+        ): either a normal completion containing ~unused~ or a throw completion
+      </h1>
+      <dl class="header">
+        <dt>description</dt>
+        <dd>It allows host environments to prevent the addition of private elements to particular host-defined exotic objects.</dd>
+      </dl>
+      <p>An implementation of HostEnsureCanAddPrivateElement must conform to the following requirements:</p>
+      <ul>
+        <li>If _O_ is not a host-defined exotic object, this abstract operation must return NormalCompletion(~unused~) and perform no other steps.</li>
+        <li>Any two calls of this abstract operation with the same argument must return the same kind of Completion Record.</li>
+      </ul>
+      <p>The default implementation of HostEnsureCanAddPrivateElement is to return NormalCompletion(~unused~).</p>
+      <p>This abstract operation is only invoked by ECMAScript hosts that are web browsers.</p>
+    </emu-clause>
+
     <emu-clause id="sec-privateget" type="abstract operation">
       <h1>
         PrivateGet (