Revert of Remove RegExp.prototype.source getter compat workaround

src/js/regexp.js

Index: src/js/regexp.js
diff --git a/src/js/regexp.js b/src/js/regexp.js
index 43565103f3a9099080dc5b6a2ab3836a70438111..a1055b3dd5e2f876dbe253dab400d53e96ee1aad 100644
--- a/src/js/regexp.js
+++ b/src/js/regexp.js
@@ -1071,6 +1071,11 @@
 // ES6 21.2.5.10.
 function RegExpGetSource() {
   if (!IS_REGEXP(this)) {
+    // TODO(littledan): Remove this RegExp compat workaround
+    if (this === GlobalRegExpPrototype) {
+      %IncrementUseCounter(kRegExpPrototypeSourceGetter);
+      return UNDEFINED;
+    }
     throw MakeTypeError(kRegExpNonRegExp, "RegExp.prototype.source");
   }
   return REGEXP_SOURCE(this);