Precompilation: don't include an object header for instructions in the text section.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 561011c9d35d09b1b48c35add36aad69e791cacc..ee67a841cb5c1918a86e0d5fda6fe5c2cdfdc1e8 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1079,10 +1079,12 @@ class RawCode : public RawObject {
   uword entry_point_;
 
   RawObject** from() {
-    return reinterpret_cast<RawObject**>(&ptr()->active_instructions_);
+    return reinterpret_cast<RawObject**>(&ptr()->instructions_);
   }
-  RawInstructions* active_instructions_;
-  RawInstructions* instructions_;
+  union {
+    RawInstructions* instructions_;
+    RawSmi* precompiled_instructions_size_;
+  };
   RawObjectPool* object_pool_;
   // If owner_ is Function::null() the owner is a regular stub.
   // If owner_ is a Class the owner is the allocation stub for that class.