Shrink AOT snapshot size and memory usage.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 7ea64d6168fec374d69eb9f804f024c701f2a563..667901b35e95e6d4f2f865845b2ddd679160b35f 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -58,6 +58,9 @@ class SkippedCodeFunctions : public ZoneAllocated {
         }
       }
 
+#if defined(DART_PRECOMPILED_RUNTIME)
+      UNREACHABLE();
+#else
       code = func->ptr()->unoptimized_code_;
       if (!code->IsMarked()) {
         // If the code wasn't strongly visited through other references
@@ -68,6 +71,7 @@ class SkippedCodeFunctions : public ZoneAllocated {
           unoptimized_code_count++;
         }
       }
+#endif
     }
     if (FLAG_log_code_drop) {
       THR_Print("  total detached current: %" Pd "\n", current_code_count);