Make StructuredClone throw if an object is not explicitly cloneable

StructuredClone used to throw for “host objects” (now objects with a [[Clone]] internal method) and
objects whose [[Class]] was not “Object” (now objects which have an internal slot other than
[[Prototype]] or [[Extensible]]). This commit restores that behavior.

This also fixes the invocation of [[Clone]] by passing memory too.

This regressed in bfb960c.

This fixes #766.

This  affects Chrome. Edge and Firefox already do the right thing.

source

@@ -7607,16 +7607,21 @@ interface <dfn>DOMStringMap</dfn> {
   <span>cloneable objects</span> and not all aspects of objects that are <span>cloneable
   objects</span> are necessarily preserved when cloned.</p>
 
-  <p>IDL-defined cloneable objects are <span>cloneable objects</span> defined through IDL that have
-  the following internal method:</p>
+  <p>IDL-defined objects have the following internal method:</p>
 
   <p>[[<dfn>Clone</dfn>]] ( <var>targetRealm</var>, <var>memory</var> )</p>
   <!-- For /deepClone/ support /memory/ would need to be changed as per StructuredClone or we would
        need to add a [[DeepClone]] abstract operation. -->
 
-  <p>IDL-defined clonable objects must define the [[<span>Clone</span>]] internal method such that
-  it either throws or returns a clone of <b>this</b>, created in <var>targetRealm</var>. It is up to
-  IDL-defined cloneable objects to define what cloning means for them.</p>
+  <p>Unless specified otherwise, invoking the [[<span>Clone</span>]] internal method must throw a
+  <code>DataCloneError</code> exception. (By default, IDL-defined objects are not <span>cloneable
+  objects</span>.)</p>
+
+  <p>IDL-defined cloneable objects are <span>cloneable objects</span> defined through IDL whose
+  [[<span>Clone</span>]] internal method is specified to run a series of steps. The result of
+  running those steps must be a thrown exception or a clone of <b>this</b>, created in
+  <var>targetRealm</var>. It is up to IDL-defined cloneable objects to define what cloning means for
+  them.</p>
 
   <p>Objects defined in the JavaScript specification are handled by the <span>StructuredClone</span>
   abstract operation directly.</p>
@@ -7640,11 +7645,15 @@ interface <dfn>DOMStringMap</dfn> {
   <!-- If we ever add /memory/ support here we need to update the language around replacing
        references -->
 
+  <p class="note">Whereas all IDL-defined objects have a [[<span>Clone</span>]] internal method, not
+  all have a [[<span>Detached</span>]] internal slot and a [[<span>Transfer</span>]] internal
+  method.</p>
+
   <p>IDL-defined transferable objects must define the [[<span>Transfer</span>]] internal method such
-  that it either throws or returns a clone of <b>this</b>, created in <var>targetRealm</var>, with
-  <b>this</b>'s underlying data shared with the return value, and <b>this</b>'s
-  [[<span>Detached</span>]] internal slot value set to true. It is up to IDL-defined transferable
-  objects to define what transfering means for them.</p>
+  that it either throws an exception or returns a clone of <b>this</b>, created in
+  <var>targetRealm</var>, with <b>this</b>'s underlying data shared with the return value, and
+  <b>this</b>'s [[<span>Detached</span>]] internal slot value set to true. It is up to IDL-defined
+  transferable objects to define what transfering means for them.</p>
 
   <p>Objects defined in the JavaScript specification are handled by the
   <span>StructuredCloneWithTransfer</span> abstract operation directly. (Technically, by
@@ -7853,17 +7862,26 @@ interface <dfn>DOMStringMap</dfn> {
     </ol>
    </li>
 
+   <li><p>Otherwise, if <var>input</var> has a [[<span>Clone</span>]] internal method, then let
+   <var>output</var> be ? <var>input</var>.[[<span>Clone</span>]](<var>targetRealm</var>,
+   <var>memory</var>).</p></li>
+
    <li><p>Otherwise, if <span>IsCallable</span>(<var>input</var>) is true, then throw a
    <code>DataCloneError</code> exception.</p></li>
 
-   <li><p>Otherwise, if <var>input</var> has an [[ErrorData]] internal slot, then throw a
-   <code>DataCloneError</code> exception.</p></li>
+   <li>
+    <p>Otherwise, if <var>input</var> has any internal slot other than [[Prototype]] or
+    [[Extensible]], then throw a <code>DataCloneError</code> exception.</p>
 
-   <li><p>Otherwise, if <var>input</var> has a [[<span>Clone</span>]] internal method, then let
-   <var>output</var> be ? <var>input</var>.[[<span>Clone</span>]](<var>targetRealm</var>).</p></li>
+    <p class="example">For instance, a [[PromiseState]] or [[WeakMapData]] internal slot.</p>
+   </li>
 
-   <li><p>Otherwise, if <var>input</var> is an exotic object, then throw a
-   <code>DataCloneError</code> exception.</p></li>
+   <li>
+    <p>Otherwise, if <var>input</var> is an exotic object, then throw a
+    <code>DataCloneError</code> exception.</p>
+
+    <p class="example">For instance, a proxy object.</p>
+  </li>
 
    <li>
     <p>Otherwise:</p>