code: function defaultWindowsLocation() {
    var files = [
      process.env['PROGRAMFILES'] || 'C:\\Program Files',
      process.env['PROGRAMFILES(X86)'] || 'C:\\Program Files (x86)'
    ].map(function(prefix) {
      return path.join(prefix, 'Mozilla Firefox\\firefox.exe');
    });
    return io.exists(files[0]).then(function(exists) {
      return exists ? files[0] : io.exists(files[1]).then(function(exists) {
        return exists ? files[1] : null;
      });
    });
  }
docs: Checks the default Windows Firefox locations in Program Files.
  @return {!promise.Promise.<?string>} A promise for the located executable.
  The promise will resolve to {@code null} if Fireox was not found.