Code coverage chokes on Wasm

[jkelleyrtp]

Problem

Our current code coverage suite currently chokes on Wasm. All our wasm_bindgen tests flip a false-positive in the code coverage system, meaning any tests we write for Wasm crates will actually bring down our reported coverage.

From what I can tell, none of the code coverage tools support this - though people have mentioned minicov even if the infra isn't really built out.

rustwasm/wasm-bindgen#2276

Questionnaire

• I'm interested in fixing this myself but don't know where to start
• I would like to fix and I have a solution
• I don't have time to fix this right now, but maybe later