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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 6dec3e4ca3e9474c7b064ad19e2184a4c3870768..af870e9595380a2424e5132a0282c8eb812e3885 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -726,6 +726,7 @@ void Isolate::FlagsInitialize(Dart_IsolateFlags* api_flags) {
   api_flags->enable_asserts = FLAG_enable_asserts;
   api_flags->enable_error_on_bad_type = FLAG_error_on_bad_type;
   api_flags->enable_error_on_bad_override = FLAG_error_on_bad_override;
+  api_flags->use_field_guards = FLAG_use_field_guards;
 }
 
 
@@ -735,6 +736,7 @@ void Isolate::FlagsCopyTo(Dart_IsolateFlags* api_flags) const {
   api_flags->enable_asserts = asserts();
   api_flags->enable_error_on_bad_type = error_on_bad_type();
   api_flags->enable_error_on_bad_override = error_on_bad_override();
+  api_flags->use_field_guards = use_field_guards();
 }
 
 
@@ -744,6 +746,7 @@ void Isolate::FlagsCopyFrom(const Dart_IsolateFlags& api_flags) {
   asserts_ = api_flags.enable_asserts;
   error_on_bad_type_ = api_flags.enable_error_on_bad_type;
   error_on_bad_override_ = api_flags.enable_error_on_bad_override;
+  use_field_guards_ = api_flags.use_field_guards;
   // Leave others at defaults.
 }
 #endif  // !defined(PRODUCT)