Disable reg-exp compilation in background via a flag

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
diff --git a/runtime/vm/flow_graph.h b/runtime/vm/flow_graph.h
index 970a7a0386c8a66289ba3d5cd08e2cb2fa0abde2..9841716092a0e2373e60b1cb04d799d2b0539510 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -285,10 +285,6 @@ class FlowGraph : public ZoneAllocated {
     return guarded_fields_;
   }
 
-  GrowableArray<const Field*>& deoptimize_dependent_code() {
-    return deoptimize_dependent_code_;
-  }
-
   ZoneGrowableArray<const LibraryPrefix*>* deferred_prefixes() const {
     return deferred_prefixes_;
   }
@@ -373,7 +369,6 @@ class FlowGraph : public ZoneAllocated {
   ZoneGrowableArray<BlockEntryInstr*>* loop_headers_;
   ZoneGrowableArray<BitVector*>* loop_invariant_loads_;
   ZoneGrowableArray<const Field*>* guarded_fields_;
-  GrowableArray<const Field*> deoptimize_dependent_code_;
   ZoneGrowableArray<const LibraryPrefix*>* deferred_prefixes_;
   DirectChainedHashMap<ConstantPoolTrait> constant_instr_pool_;
   BitVector* captured_parameters_;