Add System.register as a format target

[matthewp]

For production builds System.register is a better option long-term. I keep running into weird cases with having a bunch of named AMD defines in a single file, I don't feel confident that it's always going to be viable to go that way.

[justinbmeyer]

@matthewp I would prefer to write to AMD because its a format that's supported by other loaders. It's not "owned" by SystemJS and designed for "jspm".

But, for globals, this is something we might need to do sooner than later. I was able to get a global to work by writing out something like:

System.register("slave", [], false, function(){
    var load = {source: "window.slave = {name: 'slave'};", name: "slave", metadata: {format: "global", exports: "slave", deps: []}};
    var exportName = load.metadata.exports;

    var loader = System;

    loader.get('@@global-helpers').prepareGlobal(load.name, load.metadata.deps);

    if (exportName)
      load.source += '\nthis["' + exportName + '"] = ' + exportName + ';';

    // disable AMD detection
    var define = loader.global.define;
    loader.global.define = undefined;

    // ensure no NodeJS environment detection
    loader.global.module = undefined;
    loader.global.exports = undefined;

    loader.__exec(load);

    loader.global.define = define;

    return loader.get('@@global-helpers').retrieveGlobal(load.name, exportName, load.metadata.init);
});

instead of:

System.define("slave","window.slave = {name: 'slave'};",{"metadata": {"format": "global"}});

cc @alexisabril

[matthewp]

I got this to work for globals using the same technique in systemjs/builder. I see no reason why it has to be System.register though, this will work as a define as well. So I'm going to try this next.