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

src/x64/ic-x64.cc

Index: src/x64/ic-x64.cc
diff --git a/src/x64/ic-x64.cc b/src/x64/ic-x64.cc
index 721ae1d982aa9279dff7845b326da554459de690..fe8734caf47988adfa4c2ddb1c10c4b44e1af6b9 100644
--- a/src/x64/ic-x64.cc
+++ b/src/x64/ic-x64.cc
@@ -749,10 +749,10 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ 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);