[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 f8e4ea53d0f78cb32f63e86b30343f85c857f42c..22c51977705791e494c976da776b2bd1b34fd1ca 100644
--- a/src/ia32/ic-ia32.cc
+++ b/src/ia32/ic-ia32.cc
@@ -856,6 +856,11 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ test_b(FieldOperand(edi, Map::kBitFieldOffset),
             1 << Map::kIsAccessCheckNeeded);
   __ j(not_zero, &slow);
+  // Check that the receiver is not observed.  Observed objects are always
+  // handled by the runtime.
+  __ test_b(FieldOperand(edi, Map::kBitFieldOffset),
+            1 << Map::kIsObserved);
+  __ j(not_zero, &slow);
   // Check that the key is a smi.
   __ JumpIfNotSmi(ecx, &slow);
   __ CmpInstanceType(edi, JS_ARRAY_TYPE);