Update the toString/proxy.js to test stringifing functions

test/built-ins/Object/prototype/toString/proxy.js

 // Copyright (C) 2016 the V8 project authors. All rights reserved.
+// Copyright (C) 2016 the Apple Inc. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.
 /*---
 esid: sec-object.prototype.tostring
 es6id: 19.1.3.6
-description: Proxy of an array is treated as an array
+description: Proxy of an array/function is treated as an array/function
 info: |
   [...]
   3. Let O be ToObject(this value).
@@ -20,6 +21,14 @@ info: |
      b. Let target be the value of the [[ProxyTarget]] internal slot of
         argument.
      c. Return ? IsArray(target).
+
+  9.5.14 ProxyCreate(target, handler)
+
+  [...]
+  7. If IsCallable(target) is true, then
+    a. Set the [[Call]] internal method of P as specified in 9.5.12.
+  [...]
+
 features: [Proxy]
 ---*/
 
@@ -38,3 +47,50 @@ assert.sameValue(
   '[object Array]',
   'proxy for array proxy'
 );
+
+var functionProxy = new Proxy(function() { }, {});
+var functionProxyProxy = new Proxy(functionProxy, {});
+
+assert.sameValue(
+  Object.prototype.toString.call(functionProxy), '[object Function]', 'function proxy'
+);
+assert.sameValue(
+  Object.prototype.toString.call(functionProxyProxy),
+  '[object Function]',
+  'proxy for function proxy'
+);
+
+var arrowProxy = new Proxy(() => { }, {});
+var arrowProxyProxy = new Proxy(arrowProxy, {});
+
+assert.sameValue(
+  Object.prototype.toString.call(arrowProxy), '[object Function]', 'arrow function proxy'
+);
+assert.sameValue(
+  Object.prototype.toString.call(arrowProxyProxy),
+  '[object Function]',
+  'proxy for arrow function proxy'
+);
+
+var generatorProxy = new Proxy(function*() { }, {});
+var generatorProxyProxy = new Proxy(generatorProxy, {});
+
+assert.sameValue(
+  Object.prototype.toString.call(generatorProxy), '[object GeneratorFunction]', 'generator function proxy'
+);
+assert.sameValue(
+  Object.prototype.toString.call(generatorProxyProxy),
+  '[object GeneratorFunction]',
+  'proxy for generator function proxy'
+);
+
+delete generatorProxy.__proto__[Symbol.toStringTag];
+
+assert.sameValue(
+  Object.prototype.toString.call(generatorProxy), '[object Function]', 'generator function proxy without Symbol.toStringTag'
+);
+assert.sameValue(
+  Object.prototype.toString.call(generatorProxyProxy),
+  '[object Function]',
+  'proxy for generator function proxy without Symbol.toStringTag'
+);