Refs #25328 Use a portable shebang

sdk/bin/dartdoc

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Run dart_style/bin/format.dart on the Dart VM. This script assumes the Dart
 # SDK's directory structure.
 
 function follow_links() {
   file="$1"
   while [ -h "$file" ]; do
     # On Mac OS, readlink -f doesn't work.
     file="$(readlink "$file")"
   done
   echo "$file"
 }
 
 # Unlike $0, $BASH_SOURCE points to the absolute path of this file.
 PROG_NAME="$(follow_links "$BASH_SOURCE")"
 
 # Handle the case where dart-sdk/bin has been symlinked to.
 BIN_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
 SDK_DIR="$(cd "${BIN_DIR}/.." ; pwd -P)"
 
 SNAPSHOT="$BIN_DIR/snapshots/dartdoc.dart.snapshot"
 
 # We are running the snapshot in the built SDK.
 DART="$BIN_DIR/dart"
 exec "$DART" --packages="$BIN_DIR/snapshots/resources/dartdoc/.packages" "$SNAPSHOT" "$@"