[es6] Function length property should be configurable

test/mjsunit/es7/object-observe.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1125 matching lines @@
 //  createProxy(Proxy.create, null),
 //  createProxy(Proxy.createFunction, function(){}),
 ];
 var properties = ["a", "1", 1, "length", "setPrototype", "name", "caller"];
 
 // Cases that yield non-standard results.
 function blacklisted(obj, prop) {
   return (obj instanceof Int32Array && prop == 1) ||
          (obj instanceof Int32Array && prop === "length") ||
          (obj instanceof ArrayBuffer && prop == 1) ||
-         (obj instanceof Function && prop === "name");  // Has its own test.
+         (obj instanceof Function && prop === "name") ||  // Has its own test.
+         (obj instanceof Function && prop === "length");  // Has its own test.
 }
 
 for (var i in objects) for (var j in properties) {
   var obj = objects[i];
   var prop = properties[j];
   if (blacklisted(obj, prop)) continue;
   var desc = Object.getOwnPropertyDescriptor(obj, prop);
   print("***", typeof obj, stringifyNoThrow(obj), prop);
   if (!desc || desc.configurable)
     TestObserveConfigurable(obj, prop);
@@ skipping 662 matching lines @@
   observer.assertCallbackRecords([
     { object: fun, type: 'update', name: 'name', oldValue: 'fun' },
     { object: fun, type: 'reconfigure', name: 'name'},
     { object: fun, type: 'update', name: 'name', oldValue: 'a' },
     { object: fun, type: 'delete', name: 'name', oldValue: 'b' },
     { object: fun, type: 'add', name: 'name' },
   ]);
 })();
 
 
+(function TestFunctionLength() {
+  reset();
+
+  function fun(x) {}
+  Object.observe(fun, observer.callback);
+  fun.length = 'x';  // No change. Not writable.
+  Object.defineProperty(fun, 'length', {value: 'a'});
+  Object.defineProperty(fun, 'length', {writable: true});
+  fun.length = 'b';
+  delete fun.length;
+  fun.length = 'x';  // No change. Function.prototype.length is non writable
+  Object.defineProperty(Function.prototype, 'length', {writable: true});
+  fun.length = 'c';
+  fun.length = 'c';  // Same, no update.
+  Object.deliverChangeRecords(observer.callback);
+  observer.assertCallbackRecords([
+    { object: fun, type: 'update', name: 'length', oldValue: 1 },
+    { object: fun, type: 'reconfigure', name: 'length'},
+    { object: fun, type: 'update', name: 'length', oldValue: 'a' },
+    { object: fun, type: 'delete', name: 'length', oldValue: 'b' },
+    { object: fun, type: 'add', name: 'length' },
+  ]);
+})();
+
+
 (function TestObserveInvalidAcceptMessage() {
   var ex;
   try {
     Object.observe({}, function(){}, "not an object");
   } catch (e) {
     ex = e;
   }
   assertInstanceof(ex, TypeError);
   assertEquals("Third argument to Object.observe must be an array of strings.",
                ex.message);
 })()