fix: prevent async imports from creating chunks

[hedgepigdaniel]

Summary

Fixes #701

Test plan

[zamotany]

Thanks for the PR, looks good, but I need to check the failing tests. I suspect it's because of assertion that doesn't make sense now. Basically it was checking if the following regex is matched in the bundle:

/return asyncEval\(__webpack_require__\.p \+ "" \+ chunkId \+ "\.app1\.(ios|android)\.bundle"\);/g

but if the modules from chunk are inside the bundle, this code will never be created. Also we can probably remove code that add asyncEval helper functions.