Make sure builtins preserve guarantees about empty element array prototypes.

test/mjsunit/elide-double-hole-check-10.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: --allow-natives-syntax
+
+function f1(a, i) {
+  return a[i] + 0.5;
+}
+var arr = [,0.0,2.5];
+assertEquals(0.5, f1(arr, 1));
+assertEquals(0.5, f1(arr, 1));
+%OptimizeFunctionOnNextCall(f1);
+assertEquals(0.5, f1(arr, 1));
+
+// Trick crankshaft into accepting feedback with the array prototype
+// map even though a call on that map was never made. optopush should
+// refuse to inline the push call based on the danger that it's modifying
+// the array prototype.
+var push = Array.prototype.push;
+var array_prototype = Array.prototype;
+
+function optopush(a) {
+  push.call(a, 1);
+}
+
+function foo() {
+  optopush(array_prototype);
+}
+
+optopush([]);
+optopush([]);
+optopush([]);
+%OptimizeFunctionOnNextCall(foo);
+foo();
+assertEquals(1.5, f1(arr, 0));