Improve strictness of Annex B 3.3 for generators and async functions

test/mjsunit/harmony/sloppy-no-duplicate-async.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-async-await
+
+// Async functions don't get sloppy-mode block-scoped function hoisting
+
+// No hoisting to the global scope
+
+{
+  async function foo() {}
+  assertEquals('function', typeof foo);
+}
+assertEquals('undefined', typeof foo);
+
+// No hoisting within a function scope
+(function() {
+  { async function bar() {} }
+  assertEquals('undefined', typeof bar);
+})();
+
+// Lexical shadowing allowed, no hoisting
+(function() {
+  var y;
+  async function x() { y = 1; }
+  { async function x() { y = 2; } }
+  x();
+  assertEquals(1, y);
+})();