Reapply "Remove Code::active_instructions_."

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
diff --git a/runtime/vm/raw_object.cc b/runtime/vm/raw_object.cc
index 437ab7e7225eda6d2d7cddf17f3ec18e532cc35a..98553cd9adb70b4d858ff4aa5cd41ba695b4991a 100644
--- a/runtime/vm/raw_object.cc
+++ b/runtime/vm/raw_object.cc
@@ -535,8 +535,11 @@ intptr_t RawCode::VisitCodePointers(RawCode* raw_obj,
 
   RawCode* obj = raw_obj->ptr();
   intptr_t length = Code::PtrOffBits::decode(obj->state_bits_);
+#if defined(TARGET_ARCH_IA32)
+  // On IA32 only we embed pointers to objects directly in the generated
+  // instructions. The variable portion of a Code object describes where to
+  // find those pointers for tracing.
   if (Code::AliveBit::decode(obj->state_bits_)) {
-    // Also visit all the embedded pointers in the corresponding instructions.
     uword entry_point = reinterpret_cast<uword>(obj->instructions_->ptr()) +
         Instructions::HeaderSize();
     for (intptr_t i = 0; i < length; i++) {
@@ -546,6 +549,12 @@ intptr_t RawCode::VisitCodePointers(RawCode* raw_obj,
     }
   }
   return Code::InstanceSize(length);
+#else
+  // On all other architectures, objects are referenced indirectly through
+  // either an ObjectPool or Thread.
+  ASSERT(length == 0);
+  return Code::InstanceSize(0);
+#endif
 }