Editorial: Add some missing 'return value' invariants (tc39#1539)

... in 6.1.7.3 Invariants of the Essential Internal Methods

spec.html

@@ -1649,6 +1649,9 @@ <h2>[[HasProperty]] ( _P_ )</h2>
         </ul>
         <h2>[[Get]] ( _P_, _Receiver_ )</h2>
         <ul>
+          <li>
+            The return value must be an ECMAScript language value.
+          </li>
           <li>
             If _P_ was previously observed as a non-configurable, non-writable own data property of the target with value _V_, then [[Get]] must return the SameValue as _V_.
           </li>
@@ -1695,6 +1698,12 @@ <h2>[[OwnPropertyKeys]] ( )</h2>
             If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].
           </li>
         </ul>
+        <h2>[[Call]] ( )</h2>
+        <ul>
+          <li>
+            The return value must be an ECMAScript language value.
+          </li>
+        </ul>
         <h2>[[Construct]] ( )</h2>
         <ul>
           <li>