This is very confusing as seen in #12974 or #12906. My suggestion would be "legacy."
.pipe(rename("pdf.sandbox.min.js"))
          .pipe(gulp.dest(DIST_DIR + "build/")),
        gulp
          .src(MINIFIED_DIR + "image_decoders/pdf.image_decoders.js")
          .pipe(rename("pdf.image_decoders.min.js"))
          .pipe(gulp.dest(DIST_DIR + "image_decoders/")),
        gulp
          .src(MINIFIED_ES5_DIR + "build/pdf.js")
          .pipe(rename("pdf.min.js"))
          .pipe(gulp.dest(DIST_DIR + "es5/build/")),
        gulp
          .src(MINIFIED_ES5_DIR + "build/pdf.worker.js")
          .pipe(rename("pdf.worker.min.js"))
          .pipe(gulp.dest(DIST_DIR + "es5/build/")),
        gulp
          .src(MINIFIED_ES5_DIR + "build/pdf.sandbox.js")
          .pipe(rename("pdf.sandbox.min.js"))
          .pipe(gulp.dest(DIST_DIR + "es5/build/")),
        gulp
          .src(MINIFIED_ES5_DIR + "image_decoders/pdf.image_decoders.js")
          .pipe(rename("pdf.image_decoders.min.js"))
          .pipe(gulp.dest(DIST_DIR + "es5/image_decoders/")),
        gulp
          .src(COMPONENTS_DIR + "**/*", { base: COMPONENTS_DIR })
          .pipe(gulp.dest(DIST_DIR + "web/")),
        gulp
          .src(COMPONENTS_ES5_DIR + "**/*", { base: COMPONENTS_ES5_DIR })
          .pipe(gulp.dest(DIST_DIR + "es5/web/")),
        gulp
          .src(IMAGE_DECODERS_DIR + "**/*", { base: IMAGE_DECODERS_DIR })