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|.
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);
  };
}