webpack: Use the correct port in Casper tests.

Previously we used 9994 unconditionally, whereas we should be using
9984 or 9994 depending whether it's being run manually or via the
Casper tests.

tools/webpack

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 from __future__ import absolute_import
 
+import optparse
 import os
 import subprocess
 import sys
@@ -17,14 +18,22 @@ def run():
     subprocess.check_call(['tools/node', 'node_modules/.bin/webpack'] +
         ['--config', 'tools/webpack.production.config.js'])
 
-def run_watch():
+def run_watch(port):
     # watch and rebuild on changes, serve file from memory via webpack-dev-server
     subprocess.Popen(['tools/node', 'node_modules/.bin/webpack-dev-server'] +
-        ['--config', 'tools/webpack.config.js', '--watch-poll'])
+        ['--config', 'tools/webpack.config.js', '--watch-poll', '--port', port])
 
-if __name__ == '__main__':
-    if len(sys.argv) == 2 and sys.argv[1] == 'watch':
-        run_watch()
-    else:
-        run()
+parser = optparse.OptionParser()
+parser.add_option('--watch',
+                  action='store_true', dest='watch', default=False,
+                  help='watch for changes to source files (for development)')
+parser.add_option('--port',
+                  action='store', dest='port',
+                  default='9994', help='Set the port for the webpack server to run on')
+(options, args) = parser.parse_args()
+
+if options.watch:
+    run_watch(options.port)
+else:
+    run()