code: private Expression compileDefaultMessagePartsConstant(MsgPartsAndIds partsAndId) {
      return variables
          .addStaticField("msg_parts_" + partsAndId.id, partsToPartsList(partsAndId.parts))
          .accessor();
    }
docs: Returns an expression that evaluates to a constant {@code ImmutableList<SoyMsgPart>} used as
  the default message for when translations don't exist.
  
  <p>For each msg we generate a static final field that holds an {@code
  ImmutableList<SoyMsgPart>} which means we have to go through the somewhat awkward process of
  generating code to construct objects we have at compile time. We could do something like use
  java serialization, but just invoking the SoyMsgPart constructors isn't too hard.