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

src/ia32/ic-ia32.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 838 matching lines @@
 
   // Check that the object isn't a smi.
   __ JumpIfSmi(edx, &slow);
   // Get the map from the receiver.
   __ mov(edi, FieldOperand(edx, 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.
   __ test_b(FieldOperand(edi, Map::kBitFieldOffset),
             1 << Map::kIsAccessCheckNeeded);
   __ j(not_zero, &slow);
+  // Check that the receiver is not observed.  Observed objects are always
+  // handled by the runtime.
+  __ test_b(FieldOperand(edi, Map::kBitFieldOffset),
+            1 << Map::kIsObserved);
+  __ j(not_zero, &slow);
   // Check that the key is a smi.
   __ JumpIfNotSmi(ecx, &slow);
   __ CmpInstanceType(edi, JS_ARRAY_TYPE);
   __ j(equal, &array);
   // Check that the object is some kind of JSObject.
   __ CmpInstanceType(edi, FIRST_JS_OBJECT_TYPE);
   __ j(below, &slow);
 
   // Object case: Check key against length in the elements array.
   // eax: value
@@ skipping 789 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_IA32