VM: Fix --no-use-field-guards.

runtime/vm/jit_optimizer.cc

Index: runtime/vm/jit_optimizer.cc
diff --git a/runtime/vm/jit_optimizer.cc b/runtime/vm/jit_optimizer.cc
index c02fa8c7030b45c77316b6165ed47351c6c9a40f..3ae456168757fa44c30585cd0bbd065b170ee31f 100644
--- a/runtime/vm/jit_optimizer.cc
+++ b/runtime/vm/jit_optimizer.cc
@@ -3118,6 +3118,7 @@ bool JitOptimizer::TryInlineInstanceSetter(InstanceCallInstr* instr,
     AddReceiverCheck(instr);
   }
   if (field.guarded_cid() != kDynamicCid) {
+    ASSERT(!FLAG_use_field_guards);
     InsertBefore(instr,
                  new(Z) GuardFieldClassInstr(
                      new(Z) Value(instr->ArgumentAt(1)),
@@ -3128,6 +3129,7 @@ bool JitOptimizer::TryInlineInstanceSetter(InstanceCallInstr* instr,
   }
 
   if (field.needs_length_check()) {
+    ASSERT(!FLAG_use_field_guards);
     InsertBefore(instr,
                  new(Z) GuardFieldLengthInstr(
                      new(Z) Value(instr->ArgumentAt(1)),