[Object.observe] Don't force normalization of elements for observed objects

src/x64/stub-cache-x64.cc

 // Copyright 2012 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 1643 matching lines @@
     Handle<String> name,
     Code::StubType type) {
   // ----------- S t a t e -------------
   //  -- rcx                 : name
   //  -- rsp[0]              : return address
   //  -- rsp[(argc - n) * 8] : arg[n] (zero-based)
   //  -- ...
   //  -- rsp[(argc + 1) * 8] : receiver
   // -----------------------------------
 
-  // If object is not an array, bail out to regular call.
-  if (!object->IsJSArray() || !cell.is_null()) return Handle<Code>::null();
+  // If object is not an array or is observed, bail out to regular call.
+  if (!object->IsJSArray() ||
+      !cell.is_null() ||
+      Handle<JSArray>::cast(object)->map()->is_observed()) {
+    return Handle<Code>::null();
+  }
 
   Label miss;
   GenerateNameCheck(name, &miss);
 
   const int argc = arguments().immediate();
   StackArgumentsAccessor args(rsp, argc);
   __ movq(rdx, args.GetReceiverOperand());
 
   // Check that the receiver isn't a smi.
   __ JumpIfSmi(rdx, &miss);
@@ skipping 229 matching lines @@
     Handle<String> name,
     Code::StubType type) {
   // ----------- S t a t e -------------
   //  -- rcx                 : name
   //  -- rsp[0]              : return address
   //  -- rsp[(argc - n) * 8] : arg[n] (zero-based)
   //  -- ...
   //  -- rsp[(argc + 1) * 8] : receiver
   // -----------------------------------
 
-  // If object is not an array, bail out to regular call.
-  if (!object->IsJSArray() || !cell.is_null()) return Handle<Code>::null();
+  // If object is not an array or is observed, bail out to regular call.
+  if (!object->IsJSArray() ||
+      !cell.is_null() ||
+      Handle<JSArray>::cast(object)->map()->is_observed()) {
+    return Handle<Code>::null();
+  }
 
   Label miss, return_undefined, call_builtin;
   GenerateNameCheck(name, &miss);
 
   const int argc = arguments().immediate();
   StackArgumentsAccessor args(rsp, argc);
   __ movq(rdx, args.GetReceiverOperand());
 
   // Check that the receiver isn't a smi.
   __ JumpIfSmi(rdx, &miss);
@@ skipping 1219 matching lines @@
   // -----------------------------------
   TailCallBuiltin(masm, Builtins::kKeyedLoadIC_MissForceGeneric);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64