Allow to set a fixed port for the development server.

mojoconfig

Index: mojoconfig
diff --git a/mojoconfig b/mojoconfig
index 8be95fb8d2961869a835e526a8b7ea288980ab3b..1edace8eb31342e0df9b38142160c5ab499831c4 100644
--- a/mojoconfig
+++ b/mojoconfig
@@ -12,6 +12,11 @@
   'dev_servers': [
     {
       'host': 'https://core.mojoapps.io/',
+      # At this port the server will appear to the shell. That means actually
+      # running on this port when running a Linux shell and being forwarded from
+      # this port when running on Android. Using a fixed port enables caching
+      # server responses between shell runs.
+      'port': 31839,
       # First matching prefix will apply. Within the directiories specified for
       # a prefix, first location that contains the requested path will apply.
       'mappings': [