Changes magic sequence to '!#mojo ' and strips 'mojo ' when building url

examples/js/hello.js

-#!mojo:js_content_handler
+#!mojo mojo:js_content_handler
 // Demonstrate one JS Mojo application connecting to another to emit "hello
 // world". To run this application with mojo_shell, set DIR to be the absolute
 // path for this directory, then:
 //   mojo_shell "file://$DIR/hello.js file://$DIR/world.js"
 // Launches the Mojo hello.js application which connects to the application
 // URL specified as a Mojo application argument, world.js in this case.
 
 define("main", [
   "console",
   "mojo/services/public/js/application",
 ], function(console, application) {
 
   var Application = application.Application;
 
   class Hello extends Application {
     initialize(args) {
       console.log(this.url + ": Hello");
       if (args && args.length == 2) // args is a nullable parameter
         this.shell.connectToApplication(args[1]);
       else
         console.log("Error: expected hello.js <URL for world.js>");
       this.quit();
     }
   }
 
   return Hello;
 });