Use esm2017 bundles in unit tests

[hsubox76]

Use our esm2017 bundles in unit tests, but this requires setting the webpack TS transpile target to at least es2015 so there isn't a conflict between native classes and transpiled classes, see: https://stackoverflow.com/questions/51860043/javascript-es6-typeerror-class-constructor-client-cannot-be-invoked-without-ne

The esm2017 change is in #4168 and causes functions-exp tests to fail without the target change, when FunctionsError extends FirebaseError and calls super().

This will probably have to be changed if we begin rerunning cross-browser testing that includes IE.

Update:
Removed the mainFields change from #4168, we can discuss later. Looks like the issue occurs when functions-exp imports the util esm2017 bundle, it seems to not get transpiled by webpack or get transpiled wrong?

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Affected Products

No changes between base commit (8518b6b) and head commit (bbe9ecc).

Test Logs

• Base (8518b6b2): https://github.com/firebase/firebase-js-sdk/actions/runs/406514897
• Head (bbe9ecc7): https://github.com/firebase/firebase-js-sdk/actions/runs/406784336