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

src/arm/ic-arm.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 1414 matching lines @@
   Register elements_map = r6;
   Register elements = r9;  // Elements array of the receiver.
   // r4 and r5 are used as general scratch registers.
 
   // Check that the key is a smi.
   __ JumpIfNotSmi(key, &slow);
   // Check that the object isn't a smi.
   __ JumpIfSmi(receiver, &slow);
   // Get the map of the object.
   __ ldr(receiver_map, FieldMemOperand(receiver, 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.
   __ ldrb(ip, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
-  __ tst(ip, Operand(1 << Map::kIsAccessCheckNeeded));
+  __ tst(ip, Operand(1 << Map::kIsAccessCheckNeeded | 1 << Map::kIsObserved);
   __ b(ne, &slow);
+
   // Check if the object is a JS array or not.
   __ ldrb(r4, FieldMemOperand(receiver_map, Map::kInstanceTypeOffset));
   __ cmp(r4, Operand(JS_ARRAY_TYPE));
   __ b(eq, &array);
   // Check that the object is some kind of JSObject.
   __ cmp(r4, Operand(FIRST_JS_OBJECT_TYPE));
   __ b(lt, &slow);
 
   // Object case: Check key against length in the elements array.
   __ ldr(elements, FieldMemOperand(receiver, JSObject::kElementsOffset));
@@ skipping 232 matching lines @@
   } else {
     ASSERT(Assembler::GetCondition(branch_instr) == ne);
     patcher.EmitCondition(eq);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM