Named exports limitation - doesn't work for modules in registerRegistry

[jolyndenning]

See https://github.com/joeldenning/systemjs-issue-named-exports-registerRegistry for a demonstration of the problem.

The problem is that the the AMD extra does not call getRegister for named modules. The named-exports extra relies on getRegister to work.

The demo repo is putting two modules into the systemjs registries - the first is in the "module registry" and has the named-exports extra applied to it. The second is the named module in the "registerRegistry" and it does not have the named-exports extra applied to it. The first time that System.import() is called, it always retrieves the module from the module registry, but subsequent calls to System.import() always retrieve the module from the registerRegistry.