Normative: add globalThis (tc39#702)

 - Adds intrinsic `%GlobalThisValue%`
 - Sets it in `SetRealmGlobalObject`
 - Adds global name `globalThis`

Per tc39/proposal-global#12

spec.html

@@ -2122,6 +2122,17 @@ <h1>Well-Known Intrinsic Objects</h1>
                 The initial value of the `prototype` data property of %Generator%
               </td>
             </tr>
+            <tr>
+              <td>
+                %GlobalThisValue%
+              </td>
+              <td>
+                `global`
+              </td>
+              <td>
+                The initial value of the `globalThis` property of the global object
+              </td>
+            </tr>
             <tr>
               <td>
                 %Int8Array%
@@ -6196,11 +6207,12 @@ <h1>CreateIntrinsics ( _realmRec_ )</h1>
       <h1>SetRealmGlobalObject ( _realmRec_, _globalObj_, _thisValue_ )</h1>
       <p>The abstract operation SetRealmGlobalObject with arguments _realmRec_, _globalObj_, and _thisValue_ performs the following steps:</p>
       <emu-alg>
+        1. Let _intrinsics_ be _realmRec_.[[Intrinsics]].
         1. If _globalObj_ is *undefined*, then
-          1. Let _intrinsics_ be _realmRec_.[[Intrinsics]].
           1. Set _globalObj_ to ObjectCreate(_intrinsics_.[[%ObjectPrototype%]]).
         1. Assert: Type(_globalObj_) is Object.
         1. If _thisValue_ is *undefined*, set _thisValue_ to _globalObj_.
+        1. Set _intrinsics_.[[%GlobalThisValue%]] to _thisValue_.
         1. Set _realmRec_.[[GlobalObject]] to _globalObj_.
         1. Let _newGlobalEnv_ be NewGlobalEnvironment(_globalObj_, _thisValue_).
         1. Set _realmRec_.[[GlobalEnv]] to _newGlobalEnv_.
@@ -23984,6 +23996,12 @@ <h1>Math</h1>
       <h1>Reflect</h1>
       <p>See <emu-xref href="#sec-reflect-object"></emu-xref>.</p>
     </emu-clause>
+
+    <emu-clause id="sec-global-this">
+      <h1>globalThis</h1>
+      <p>The initial value of `globalThis` is the well-known intrinsic object <dfn>%GlobalThisValue%</dfn>.</p>
+      <p>This property has the attributes { [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.</p>
+    </emu-clause>
   </emu-clause>
 </emu-clause>