code: private Node tryFoldAddConstantString(Node n, Node left, Node right) {
      if (left.isString() || right.isString()
          || left.isArrayLit() || right.isArrayLit()) {
        // Add strings.
        String leftString = NodeUtil.getStringValue(left);
        String rightString = NodeUtil.getStringValue(right);
        if (leftString != null && rightString != null) {
          Node newStringNode = IR.string(leftString + rightString);
          n.replaceWith(newStringNode);
          reportChangeToEnclosingScope(newStringNode);
          return newStringNode;
        }
      }
  
      return n;
    }
docs: Try to fold an ADD node with constant operands