[dart2js] Implement function tearoff equality corner cases

[eernstg]

This is the dart2js specific version of #46834, which has the details.

[sigmundch]

@fishythefish - is there much left to do in dart2js?

[fishythefish]

Yes, I need to finish work on https://dart-review.googlesource.com/c/sdk/+/205764. (It's not a lot of work; I just need to actually get back to it.)

[fishythefish]

Closing since https://dart-review.googlesource.com/c/sdk/+/210981 landed.