Make integer indexed properties ("elements") work for proxies.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index a76f3b6e81fc408721ed5e2777b8891a1aa7dbd1..ad5236968487789146c532482c896aedce6d3be2 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -354,7 +354,7 @@ function IN(x) {
   if (!IS_SPEC_OBJECT(x)) {
     throw %MakeTypeError('invalid_in_operator_use', [this, x]);
   }
-  return %_IsNonNegativeSmi(this) && !%IsJSProxy(x) ?
+  return %_IsNonNegativeSmi(this) ?
     %HasElement(x, this) : %HasProperty(x, %ToString(this));
 }