Fix arrow functions requiring context without slots.

test/mjsunit/harmony/regress/regress-4160.js

+// Copyright 2015 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-arrow-functions --allow-natives-syntax
+
+(function(x) {
+  (function(x) {
+    var boom = (() => eval(x));
+    assertEquals(23, boom());
+    assertEquals(23, boom());
+    %OptimizeFunctionOnNextCall(boom);
+    assertEquals(23, boom());
+    assertEquals("23", x);
+  })("23");
+  assertEquals("42", x);
+})("42");
+
+(function(x) {
+  (function(x) {
+    var boom = (() => (eval("var x = 66"), x));
+    assertEquals(66, boom());
+    assertEquals(66, boom());
+    %OptimizeFunctionOnNextCall(boom);
+    assertEquals(66, boom());
+    assertEquals("23", x);
+  })("23");
+  assertEquals("42", x);
+})("42");