code: void appendString(JCTree tree) {
              Type t = tree.type.baseType();
              if (!t.isPrimitive() && t.tsym != syms.stringType.tsym) {
                  t = syms.objectType;
              }
              items.makeMemberItem(getStringBufferAppend(tree, t), false).invoke();
          }
docs: Append value (on tos) to string buffer (on tos - 1).