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

src/ia32/ic-ia32.cc

Index: src/ia32/ic-ia32.cc
diff --git a/src/ia32/ic-ia32.cc b/src/ia32/ic-ia32.cc
index 0b7c4a828bce881c03ed05d44cfbf3943e103164..dab9dd7a447ae089d87419557feba98cce0c4e43 100644
--- a/src/ia32/ic-ia32.cc
+++ b/src/ia32/ic-ia32.cc
@@ -874,10 +874,10 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ 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.
+  // 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.
   __ test_b(FieldOperand(edi, Map::kBitFieldOffset),
-            1 << Map::kIsAccessCheckNeeded);
+            1 << Map::kIsAccessCheckNeeded | 1 << Map::kIsObserved);
   __ j(not_zero, &slow);
   // Check that the key is a smi.
   __ JumpIfNotSmi(ecx, &slow);