Currently we don't allow two mojoms to import each other. We don't support forward declaration of types either.

That is annoying in some cases. The following is one example:

https://code.google.com/p/chromium/codesearch#chromium/src/services/shell/public/interfaces/connector.mojom&l=74

This file defines the following struct:
struct ClientProcessConnection {
  // Provides the shell the ability to bind a ShellClient from the client
  // process to the instance it creates.
  handle<message_pipe> shell_client;

  // Allows the client process launcher to tell the shell the PID of the process
  // it created (the pid isn't supplied directly here as the process may not
  // have been launched by the time Connect() is called.)
  handle<message_pipe> pid_receiver_request;
};

It uses raw handle<message_pipe> in order to avoid importing the mojom files containing those interfaces because that would result in circular dependencies.

We need to either:
(1) allow circular imports in mojoms, but generate code that still compiles (no circular #include, etc.)
(2) introduce syntax for forward declaration