message: Compare against undefined, not string undefined.
diff: @@ -17,7 +17,7 @@ import * as stream from 'stream';
  import File = require('vinyl');
  -if ((Symbol as any).asyncIterator !== 'undefined') {
  +if ((Symbol as any).asyncIterator === undefined) {
  (Symbol as any).asyncIterator = Symbol('asyncIterator');
  }
  