[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 15f410c1345e8bbd4dc7db6a46494fa357763ed6..8ccf7e38fb6e58c2b46e97c78786a40b371c9ab8 100644
--- a/src/x64/ic-x64.cc
+++ b/src/x64/ic-x64.cc
@@ -730,6 +730,11 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ testb(FieldOperand(r9, Map::kBitFieldOffset),
            Immediate(1 << Map::kIsAccessCheckNeeded));
   __ j(not_zero, &slow_with_tagged_index);
+  // Check that the receiver is not observed.  Observed objects are always
+  // handled by the runtime.
+  __ testb(FieldOperand(r9, Map::kBitFieldOffset),
+           Immediate(1 << Map::kIsObserved));
+  __ j(not_zero, &slow_with_tagged_index);
   // Check that the key is a smi.
   __ JumpIfNotSmi(rcx, &slow_with_tagged_index);
   __ SmiToInteger32(rcx, rcx);