Fix product build

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index f058cd9f7d20daed7abd881810e164c67439e688..7ede2f56112211b834925a6121a8a79e98436a98 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1055,6 +1055,9 @@ bool Isolate::IsPaused() const {
 
 
 void Isolate::PausePostRequest() {
+  if (!FLAG_support_debugger) {
+    return;
+  }
   if (debugger_ == NULL) {
     return;
   }