VM: Allow configuring use_field_guards on the per-isolate basis and include it into snapshot featur…

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 153299f0b57185384843562f7bd5a759c00b5216..3d9bf2ba68935a70fb5ec9ea17a3dc3f71bdd013 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -638,11 +638,13 @@ class Isolate : public BaseIsolate {
   bool asserts() const { return FLAG_enable_asserts; }
   bool error_on_bad_type() const { return FLAG_error_on_bad_type; }
   bool error_on_bad_override() const { return FLAG_error_on_bad_override; }
+  bool use_field_guards() const { return FLAG_use_field_guards; }
 #else   // defined(PRODUCT)
   bool type_checks() const { return type_checks_; }
   bool asserts() const { return asserts_; }
   bool error_on_bad_type() const { return error_on_bad_type_; }
   bool error_on_bad_override() const { return error_on_bad_override_; }
+  bool use_field_guards() const { return use_field_guards_; }
 #endif  // defined(PRODUCT)
 
   static void KillAllIsolates(LibMsgId msg_id);
@@ -772,6 +774,7 @@ class Isolate : public BaseIsolate {
   bool asserts_;
   bool error_on_bad_type_;
   bool error_on_bad_override_;
+  bool use_field_guards_;
 #endif  // !defined(PRODUCT)
 
   // Timestamps of last operation via service.