Move third_party/firefox_jsshell dependency to cloud storage

tools/testing/bin/jsshell

 #!/bin/bash
 # Copyright (c) 2012, 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.
 
+JSDIR=${0%/*}/../../../third_party/firefox_jsshell
 
 case $OSTYPE in
     *[Dd]arwin*)
-        exec ${0%/*}/../../../third_party/firefox_jsshell/macos/js "$@"
+        exec $JSDIR/mac/jsshell/js "$@"
         ;;
     *[Ll]inux*)
-        LD_LIBRARY_PATH=${0%/*}/../../../third_party/firefox_jsshell/linux/ \
-            exec ${0%/*}/../../../third_party/firefox_jsshell/linux/js "$@"
+        LD_LIBRARY_PATH=$JSDIR/linux/jsshell/ \
+            exec $JSDIR/linux/jsshell/js "$@"
         ;;
     *)
         echo No jsshell binary for $(uname) >2
         exit 1
         ;;
 esac