code: function uglifyWithCopyrights() {
      const preserveComments = (f) => {
          return (_node, comment) => {
              const text = comment.value;
              const type = comment.type;
              if (/@minifier_do_not_preserve/.test(text)) {
                  return false;
              }
              const isOurCopyright = IS_OUR_COPYRIGHT_REGEXP.test(text);
              if (isOurCopyright) {
                  if (f.__hasOurCopyright) {
                      return false;
                  }
                  f.__hasOurCopyright = true;
                  return true;
              }
              if ('comment2' === type) {
                  // check for /*!. Note that text doesn't contain leading /*
                  return (text.length > 0 && text[0] === '!') || /@preserve|license|@cc_on|copyright/i.test(text);
              }
              else if ('comment1' === type) {
                  return /license|copyright/i.test(text);
              }
              return false;
          };
      };
      const minify = composer(uglifyes);
      const input = es.through();
      const output = input
          .pipe(flatmap((stream, f) => {
          return stream.pipe(minify({
              output: {
                  comments: preserveComments(f),
                  max_line_len: 1024
              }
          }));
      }));
      return es.duplex(input, output);
  }
docs: Wrap around uglify and allow the preserveComments function
  to have a file "context" to include our copyright only once per file.