Replace __proto__ with non-Annex B alternative

test/language/expressions/object/method-definition/async-super-call-body.js

@@ -16,11 +16,12 @@ var sup = {
 }
 
 var child = {
-  __proto__: sup,
   async method() {
     var x = await super.method();
     assert.sameValue(x, 'sup');
   }
 }
 
+Object.setPrototypeOf(child, sup);
+
 child.method().then($DONE, $DONE);

test/language/expressions/object/method-definition/async-super-call-param.js

@@ -16,12 +16,13 @@ var sup = {
 }
 
 var child = {
-  __proto__: sup,
   async method(x = super.method()) {
     var y = await x;
     assert.sameValue(y, 'sup');
   }
 }
 
+Object.setPrototypeOf(child, sup);
+
 child.method().then($DONE, $DONE);