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

src/mips/ic-mips.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 1311 matching lines @@
   __ JumpIfNotSmi(key, &slow);
   // Check that the object isn't a smi.
   __ JumpIfSmi(receiver, &slow);
   // Get the map of the object.
   __ lw(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.
   __ lbu(t0, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
   __ And(t0, t0, Operand(1 << Map::kIsAccessCheckNeeded));
   __ Branch(&slow, ne, t0, Operand(zero_reg));
+  // Check that the receiver is not observed.  Observed objects are always
+  // handled by the runtime.
+  __ lbu(t0, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
+  __ And(t0, t0, Operand(1 << Map::kIsObserved));
+  __ Branch(&slow, ne, t0, Operand(zero_reg));
   // Check if the object is a JS array or not.
   __ lbu(t0, FieldMemOperand(receiver_map, Map::kInstanceTypeOffset));
   __ Branch(&array, eq, t0, Operand(JS_ARRAY_TYPE));
   // Check that the object is some kind of JSObject.
   __ Branch(&slow, lt, t0, Operand(FIRST_JS_OBJECT_TYPE));
 
   // Object case: Check key against length in the elements array.
   __ lw(elements, FieldMemOperand(receiver, JSObject::kElementsOffset));
   // Check array bounds. Both the key and the length of FixedArray are smis.
   __ lw(t0, FieldMemOperand(elements, FixedArray::kLengthOffset));
@@ skipping 317 matching lines @@
   } else {
     ASSERT(Assembler::IsBne(branch_instr));
     patcher.ChangeBranchCondition(eq);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS