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

src/x64/ic-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 731 matching lines @@
   //  -- rsp[0] : return address
   // -----------------------------------
   Label slow, slow_with_tagged_index, fast_object, fast_object_grow;
   Label fast_double, fast_double_grow;
   Label array, extra, check_if_double_array;
 
   // Check that the object isn't a smi.
   __ JumpIfSmi(rdx, &slow_with_tagged_index);
   // Get the map from the receiver.
   __ movq(r9, FieldOperand(rdx, HeapObject::kMapOffset));
-  // Check that the receiver does not require access checks.  We need
-  // to do this because this generic stub does not perform map checks.
+  // Check that the receiver does not require access checks and is not observed.
+  // The generic stub does not perform map checks or handle observed objects.
   __ testb(FieldOperand(r9, Map::kBitFieldOffset),
-           Immediate(1 << Map::kIsAccessCheckNeeded));
+           Immediate(1 << Map::kIsAccessCheckNeeded | 1 << Map::kIsObserved));
   __ j(not_zero, &slow_with_tagged_index);
   // Check that the key is a smi.
   __ JumpIfNotSmi(rcx, &slow_with_tagged_index);
   __ SmiToInteger32(rcx, rcx);
 
   __ CmpInstanceType(r9, JS_ARRAY_TYPE);
   __ j(equal, &array);
   // Check that the object is some kind of JSObject.
   __ CmpInstanceType(r9, FIRST_JS_OBJECT_TYPE);
   __ j(below, &slow);
@@ skipping 936 matching lines @@
   Condition cc = (check == ENABLE_INLINED_SMI_CHECK)
       ? (*jmp_address == Assembler::kJncShortOpcode ? not_zero : zero)
       : (*jmp_address == Assembler::kJnzShortOpcode ? not_carry : carry);
   *jmp_address = static_cast<byte>(Assembler::kJccShortPrefix | cc);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64