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

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 8d147a14721047d5fe0eb891d3c62552c4dc148e..f0c37f2b9ecf2aa10c18d74d1bc972d428f5f433 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2330,6 +2330,15 @@ MaybeObject* Heap::NumberToString(Object* number,
 }
 
 
+MaybeObject* Heap::Uint32ToString(uint32_t value,
+                                  bool check_number_string_cache) {
+  Object* number;
+  MaybeObject* maybe = NumberFromUint32(value);
+  if (!maybe->To<Object>(&number)) return maybe;
+  return NumberToString(number, check_number_string_cache);
+}
+
+
 Map* Heap::MapForExternalArrayType(ExternalArrayType array_type) {
   return Map::cast(roots_[RootIndexForExternalArrayType(array_type)]);
 }