fix: do not break Import Maps

[hronro]

Fix #464

[apollo-cla]

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[MrDoomBringer]

Hey @hronro, thanks a ton for the PR!

I went ahead and added a commit that waits for the DOMContentLoaded event before injecting the script, just to be sure we have access to the importmap script if it's available. Tested on https://generator.jspm.io/, the AC fullstack tutorial (w/ apollo/[email protected]), and the DevTools example app (apollo/[email protected])

Will get this merged shortly pending a quick once-over

All the best, Emmanuel :-)