[proxies] Improve error messages.

src/js/proxy.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.
 
 (function(global, utils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 18 matching lines @@
 // Proxy Builtins
 
 // Implements part of ES6 9.5.11 Proxy.[[Enumerate]]:
 // Call the trap, which should return an iterator, exhaust the iterator,
 // and return an array containing the values.
 function ProxyEnumerate(trap, handler, target) {
   // 7. Let trapResult be ? Call(trap, handler, «target»).
   var trap_result = %_Call(trap, handler, target);
   // 8. If Type(trapResult) is not Object, throw a TypeError exception.
   if (!IS_SPEC_OBJECT(trap_result)) {
-    throw MakeTypeError(kProxyTrapReturnedNonObject, handler, trap_result,
-                        "enumerate");
+    throw MakeTypeError(kProxyEnumerateNonObject);
   }
   // 9. Return trapResult.
   var result = [];
   for (var it = trap_result.next(); !it.done; it = trap_result.next()) {
     var key = it.value;
     // Not yet spec'ed as of 2015-11-25, but will be spec'ed soon:
     // If the iterator returns a non-string value, throw a TypeError.
     if (!IS_STRING(key)) {
-      throw MakeTypeError(kProxyTrapReturned, handler, "non-String",
-                          "enumerate");
+      throw MakeTypeError(kProxyEnumerateNonString);
     }
     result.push(key);
   }
   return result;
 }
 
 //-------------------------------------------------------------------
 
 //Set up non-enumerable properties of the Proxy object.
 utils.InstallFunctions(GlobalProxy, DONT_ENUM, [
   "revocable", ProxyCreateRevocable
 ]);
 
 // -------------------------------------------------------------------
 // Exports
 
 %InstallToContext([
   "proxy_enumerate", ProxyEnumerate,
 ]);
 
 })