Revert "Enable Object.observe by default"

src/accessors.cc

Index: src/accessors.cc
diff --git a/src/accessors.cc b/src/accessors.cc
index 00ff201a744df7e194ab541602a9f681057735b3..9bc728d3116f4349dc1fd4bcfb42003aa292d382 100644
--- a/src/accessors.cc
+++ b/src/accessors.cc
@@ -600,7 +600,10 @@ MaybeObject* Accessors::FunctionSetPrototype(Isolate* isolate,
   }
 
   Handle<Object> old_value;
-  bool is_observed = *function == *object && function->map()->is_observed();
+  bool is_observed =
+      FLAG_harmony_observation &&
+      *function == *object &&
+      function->map()->is_observed();
   if (is_observed) {
     if (function->has_prototype())
       old_value = handle(function->prototype(), isolate);