[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 aa2773462c73266aa2d61a07f1635e13bd25f825..4082e4a054ad028caa64919570bc7c32108de7aa 100644
--- a/src/mips/ic-mips.cc
+++ b/src/mips/ic-mips.cc
@@ -1329,6 +1329,11 @@ void KeyedStoreIC::GenerateGeneric(MacroAssembler* masm,
   __ lbu(t0, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
   __ And(t0, t0, Operand(1 << Map::kIsAccessCheckNeeded));
   __ Branch(&slow, ne, t0, Operand(zero_reg));
+  // Check that the receiver is not observed.  Observed objects are always
+  // handled by the runtime.
+  __ lbu(t0, FieldMemOperand(receiver_map, Map::kBitFieldOffset));
+  __ And(t0, t0, Operand(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));
   __ Branch(&array, eq, t0, Operand(JS_ARRAY_TYPE));