Multiple self-executing bundles fail

[evanweible-wf]

If you try to include more than one sfx bundle on a page, only the first will execute correctly.

From what I can tell, the first sfx bundle leaves a global reference to System when it executes. This is an issue because later sfx bundles will check for a global System, and if found, will try to use it to import instead of self-executing. This then fails because System is not actually a SystemJS implementation but rather a shim used in the sfx bundling and thus lacks the .import() method.

I've created a gist with a reduced test case that you can use to try this out for yourself: https://gist.github.com/evanweible-wf/c85adae40b622477ba91

[guybedford]

Moving to systemjs/builder#13.