code: void appendStrings(JCTree tree) {
              tree = TreeInfo.skipParens(tree);
              if (tree.hasTag(PLUS) && tree.type.constValue() == null) {
                  JCBinary op = (JCBinary) tree;
                  if (op.operator.kind == MTH &&
                      ((OperatorSymbol) op.operator).opcode == string_add) {
                      appendStrings(op.lhs);
                      appendStrings(op.rhs);
                      return;
                  }
              }
              genExpr(tree, tree.type).load();
              appendString(tree);
          }
docs: Add all strings in tree to string buffer.