This is very confusing as seen in #12974 or #12906. My suggestion would be "legacy."
return merge([
        createMainBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createWorkerBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createSandboxBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createWebBundle(defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
        ),
        gulp
          .src(COMMON_WEB_FILES, { base: "web/" })
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        gulp
          .src(
            ["web/locale/*/viewer.properties", "web/locale/locale.properties"],
            { base: "web/" }
          )
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        gulp
          .src(["external/bcmaps/*.bcmap", "external/bcmaps/LICENSE"], {
            base: "external/bcmaps",
          })
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web/cmaps")),
        preprocessHTML("web/viewer.html", defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
        ),