[vm/compiler] Use Isolate::use_field_guards instead of FLAG_use_field_guards.
[vm/compiler] Use Isolate::use_field_guards instead of FLAG_use_field_guards.

These two values are not necessarily in sync: FLAG_use_field_guards can be set to
false while kernel-service is running from an app-jit snapshot with field
guards.

This caused major flakiness on kernel opt-counter-threshold bot.

This was a bug introduced by refactoring in d6eb8c6c27c7a197a8e48b0e3e0baa28518c80ea.

Change-Id: Icba874d319c1e96df90cae2d81e0bce3e752808b
Reviewed-on: https://dart-review.googlesource.com/38620
Reviewed-by: Erik Corry <b520ba1e7f871f909b0b4a9e1cf9a391adc0e954@google.com>

dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk