Don't fast-store to observed objects.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 074ca17fe101efbf5ceb08f4dcfceb16cdaadf11..eb12ea4c8e63049065ff2380cf13edaba880cc8c 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -4774,6 +4774,7 @@ static bool ComputeLoadStoreField(Handle<Map> type,
                                   Handle<String> name,
                                   LookupResult* lookup,
                                   bool is_store) {
+  if (is_store && type->is_observed()) return false;
   if (type->has_named_interceptor()) {
     lookup->InterceptorResult(NULL);
     return false;
@@ -5061,6 +5062,7 @@ bool HOptimizedGraphBuilder::TryStorePolymorphicAsMonomorphic(
     Handle<Map> map = types->at(count);
     // Pass false to ignore transitions.
     if (!ComputeLoadStoreField(map, name, &lookup, false)) break;
+    if (map->is_observed()) return false;
 
     HObjectAccess new_access = HObjectAccess::ForField(map, &lookup, name);
     Representation new_representation =