Fix the context scope of SystemJS.register when called from browsers System.register

[joeljeske]

The SystemJS.register requires the correct this. If modules are registered using System.register (as opposed to SystemJS.register) AND there exists another global.System object, the register module call fails, in my case the error was:

register-loader.js:456 Uncaught TypeError: Cannot set property 'lastRegister' of undefined
    at Object.RegisterLoader$1.register (register-loader.js:456)
    at Object.systemJSPrototype.register (systemjs-production-loader.js:206)
    at Object.envGlobal.System.register (system-production.js:22)
    at main.js:1

[guybedford]

Thanks!