Remove the debugger_ field from Isolate in a PRODUCT build

runtime/lib/developer.cc

Index: runtime/lib/developer.cc
diff --git a/runtime/lib/developer.cc b/runtime/lib/developer.cc
index c314aaa1d12ebed5d38f68f78628528a494be03a..f116b5cae3d57919563e67d68378671a9f71a399 100644
--- a/runtime/lib/developer.cc
+++ b/runtime/lib/developer.cc
@@ -21,6 +21,7 @@ namespace dart {
 // Native implementations for the dart:developer library.
 DEFINE_NATIVE_ENTRY(Developer_debugger, 2) {
   GET_NON_NULL_NATIVE_ARGUMENT(Bool, when, arguments->NativeArgAt(0));
+#if !defined(PRODUCT)
   GET_NATIVE_ARGUMENT(String, msg, arguments->NativeArgAt(1));
   Debugger* debugger = isolate->debugger();
   if (!FLAG_support_debugger || !debugger) {
@@ -29,6 +30,7 @@ DEFINE_NATIVE_ENTRY(Developer_debugger, 2) {
   if (when.value()) {
     debugger->PauseDeveloper(msg);
   }
+#endif
   return when.raw();
 }