Optimize supportsNativeFetch() with a fast path that avoids DOM I/O

[AaronO]

This fixes #2324 by adding a fast path for the scenario where the browser supports fetch and window.fetch hasn't been wrapped or polyfilled (which should be very common on modern browsers)

This avoids expensive DOM I/O in the critical path during the initial load.

Fallbacking to the expensive iframe check, if necessary.

Note

The new isNativeFetch() check is also more robust than the previous implementation, which could yield false positives (since it only checked if the function's body contained native).

[HazAT]

Thanks for the issue report + nice PR!
LGTM wdyt @kamilogorek

[kamilogorek]

Looks much better now. Thanks! Feel free to merge @HazAT

[AaronO]

@HazAT Thanks for merging this, any idea of when it will be shipped in a release ?

[HazAT]

We are in the process of releasing 5.10.0 with a lot of internal improvements.
We are testing it on sentry.io, I would say mid next week.