Remove the debugger_ field from Isolate in a PRODUCT build

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index e5b1849d6999df4c3956dc5d275fd7fd6ee3ba6a..ee6201d68e220eee3c455d98e9e4003cf5e82878 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -319,6 +319,7 @@ class Isolate : public BaseIsolate {
   }
   Mutex* megamorphic_lookup_mutex() const { return megamorphic_lookup_mutex_; }
 
+#if !defined(PRODUCT)
   Debugger* debugger() const {
     if (!FLAG_support_debugger) {
       return NULL;
@@ -326,6 +327,7 @@ class Isolate : public BaseIsolate {
     ASSERT(debugger_ != NULL);
     return debugger_;
   }
+#endif
 
   void set_single_step(bool value) { single_step_ = value; }
   bool single_step() const { return single_step_; }
@@ -766,7 +768,7 @@ class Isolate : public BaseIsolate {
   Dart_EnvironmentCallback environment_callback_;
   Dart_LibraryTagHandler library_tag_handler_;
   ApiState* api_state_;
-  Debugger* debugger_;
+  NOT_IN_PRODUCT(Debugger* debugger_);
   bool resume_request_;
   int64_t last_resume_timestamp_;
   Random random_;