Implement `in' for proxies.

src/runtime.js

 // Copyright 2006-2008 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 336 matching lines @@
 function DELETE(key, strict) {
   return %DeleteProperty(%ToObject(this), %ToString(key), strict);
 }
 
 
 // ECMA-262, section 11.8.7, page 54.
 function IN(x) {
   if (!IS_SPEC_OBJECT(x)) {
     throw %MakeTypeError('invalid_in_operator_use', [this, x]);
   }
-  return %_IsNonNegativeSmi(this) ? %HasElement(x, this) : %HasProperty(x, %ToString(this));
+  return %_IsNonNegativeSmi(this) && !%IsJSProxy(x) ?
+    %HasElement(x, this) : %HasProperty(x, %ToString(this));
 }
 
 
 // ECMA-262, section 11.8.6, page 54. To make the implementation more
 // efficient, the return value should be zero if the 'this' is an
 // instance of F, and non-zero if not. This makes it possible to avoid
 // an expensive ToBoolean conversion in the generated code.
 function INSTANCE_OF(F) {
   var V = this;
   if (!IS_FUNCTION(F)) {
@@ skipping 272 matching lines @@
   throw %MakeTypeError('cannot_convert_to_primitive', []);
 }
 
 
 // NOTE: Setting the prototype for Array must take place as early as
 // possible due to code generation for array literals.  When
 // generating code for a array literal a boilerplate array is created
 // that is cloned when running the code.  It is essential that the
 // boilerplate gets the right prototype.
 %FunctionSetPrototype($Array, new $Array(0));