Invalid implementation of map in Lesson 4 will break code navigation

[jtrinklein]

CodeMirror checks for and uses map on Array.prototype if it is defined. see it on codemirror.js L7213

  function map(array, f) {
    var out = [];
    for (var i = 0; i < array.length; i++) out[i] = f(array[i], i);
    return out;
  }
  if ([].map) map = function(array, f) { return array.map(f); };

solvable by commenting out/removing line 7213

  function map(array, f) {
    var out = [];
    for (var i = 0; i < array.length; i++) out[i] = f(array[i], i);
    return out;
  }
-  if ([].map) map = function(array, f) { return array.map(f); };

[morenoh149]

could you submit this as a PR