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

src/mips/ic-mips.cc

Index: src/mips/ic-mips.cc
diff --git a/src/mips/ic-mips.cc b/src/mips/ic-mips.cc
index 0fe044a6c5a02dfc66fa51b02748672122a61eba..c7e1a2ada602392c952118d8f36162a37e67682b 100644
--- a/src/mips/ic-mips.cc
+++ b/src/mips/ic-mips.cc
@@ -1354,10 +1354,11 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ JumpIfSmi(receiver, &slow);
   // Get the map of the object.
   __ lw(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.
   __ lbu(t0, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
-  __ And(t0, t0, Operand(1 << Map::kIsAccessCheckNeeded));
+  __ And(t0, t0, Operand(1 << Map::kIsAccessCheckNeeded |
+                         1 << Map::kIsObserved));
   __ Branch(&slow, ne, t0, Operand(zero_reg));
   // Check if the object is a JS array or not.
   __ lbu(t0, FieldMemOperand(receiver_map, Map::kInstanceTypeOffset));