code: function makeDefine(mapping, id) {
    var require = function(id) { return mapping[id]; };
    var exports = mapping[id] = {};
    var module = null; // Unused arg. Included for completeness.
    return function(factory) {
      factory(require, exports, module);
    };
  }
docs: The official 'source-map' requires a 'define' function that matches the CommonJS module specification. Since it only uses a subset of that functionality, we cheat here and only provide that minimum subset. 
  @param {Object} mapping Maps between module ids and exports.
  @param {string} id A module id.
  @return {Function} A 'define' function that initializes |mapping[id]| with
  the exports from |factory|.