Normative: make SharedArrayBuffer optional (tc39#1903)

spec.html

@@ -37458,12 +37458,14 @@ <h1>The SharedArrayBuffer Constructor</h1>
       <p>The SharedArrayBuffer constructor:</p>
       <ul>
         <li>is <dfn>%SharedArrayBuffer%</dfn>.</li>
-        <li>is the initial value of the *"SharedArrayBuffer"* property of the global object.</li>
+        <li>is the initial value of the *"SharedArrayBuffer"* property of the global object, if that property is present (see below).</li>
         <li>creates and initializes a new SharedArrayBuffer object when called as a constructor.</li>
         <li>is not intended to be called as a function and will throw an exception when called in that manner.</li>
         <li>is designed to be subclassable. It may be used as the value of an `extends` clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a `super` call to the SharedArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the `SharedArrayBuffer.prototype` built-in methods.</li>
       </ul>
 
+      <p>Whenever a host does not provide concurrent access to SharedArrayBuffer objects it may omit the *"SharedArrayBuffer"* property of the global object.</p>
+
       <emu-note>
         <p>Unlike an `ArrayBuffer`, a `SharedArrayBuffer` cannot become detached, and its internal [[ArrayBufferData]] slot is never *null*.</p>
       </emu-note>