js-sys: add Object.fromEntries

[bakkot]

Fixes #1453.

I couldn't actually get tests to pass, possibly because Node does not yet support this function.

[bakkot]

Yeah, looks like CI has the same problem as I did locally. I'd appreciate input from a maintainer here - to my (uneducated) eye, it looks like tests in CI are being run in Firefox, but Firefox has had support for Object.fromEntries since version 63 (released in October 2018) and Azure's Ubuntu agent claims claims to be running Firefox 66.

[alexcrichton]

Oh so crates/js-sys/tests/wasm/* is actually all executed in Node.js which may explain the discrepancy here, but can you try adding the tests to crates/js-sys/tests/headless.rs as those should be run in Firefox on Azure?

[bakkot]

Hm. I believe Node 12 will include v8 7.4, which includes Object.fromEntries. It's due to be cut on April 23. So maybe it'd be simpler to just wait for that?

[alexcrichton]

Seems reasonable to me!

[alexcrichton]

I just updated to node 12 in #1484, want to rebase this and see if it works?

[bakkot]

@alexcrichton looks like it worked! (Once I fixed the silly bug in the test, anyway.)