docs: public ExprNode valueAsExpr(ErrorReporter reporter) {
      checkState(value == null);
      if (valueExprList.size() > 1) {
        reporter.report(
            valueExprList.get(1).getSourceLocation(), EXPECTED_A_SINGLE_EXPRESSION, key.identifier());
        // Return the first expr to avoid an NPE in CallNode ctor.
        return valueExprList.get(0);
      }
      return Iterables.getOnlyElement(valueExprList);
    }