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

src/arm/ic-arm.cc

Index: src/arm/ic-arm.cc
diff --git a/src/arm/ic-arm.cc b/src/arm/ic-arm.cc
index 025a590f0cba202b9636b88d03626deea2e371b6..8c41224686072bd1f0e08856fb794f533e8878d3 100644
--- a/src/arm/ic-arm.cc
+++ b/src/arm/ic-arm.cc
@@ -1432,11 +1432,12 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ JumpIfSmi(receiver, &slow);
   // Get the map of the object.
   __ ldr(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.
+  // 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.
   __ ldrb(ip, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
-  __ tst(ip, Operand(1 << Map::kIsAccessCheckNeeded));
+  __ tst(ip, Operand(1 << Map::kIsAccessCheckNeeded | 1 << Map::kIsObserved);
   __ b(ne, &slow);
+
   // Check if the object is a JS array or not.
   __ ldrb(r4, FieldMemOperand(receiver_map, Map::kInstanceTypeOffset));
   __ cmp(r4, Operand(JS_ARRAY_TYPE));