message: Improve the assertion message about self-closing JSX
  For example: `AssertionError [ERR_ASSERTION]: unexpected closingElement element in self-closing JSXElement`
diff: @@ -1162,7 +1162,10 @@ function genericPrintNoParens(path, options, print) {
  var openingLines = path.call(print, openingPropName);
  if (n[openingPropName].selfClosing) {
  - assert.ok(!n[closingPropName]);
  + assert.ok(
  + !n[closingPropName],
  + "unexpected " + closingPropName + " element in self-closing " + n.type
  + );
  return openingLines;
  }
  