constructor(src, options) {
    this._opts = {flow: {}, map: {}, prefix: './', ...options};
    this._src = src;
    this._idx = 0; // Tracks our current substring index in src
    this._dst = ''; // destination for new rewritten source
  }
Runs the flow-parser on a given JavaScript file and rewrites modules
according to rewrite rules passed into options:

 prefix: module prefix defaults to './', (not used when modules are mapped)
    map: module => rewriteModule map
   flow: options to pass to the flow-parser on parse(...)