The patch to be attached will let wasm modules import JS functions that return multiple values.

Spec here: https://webassembly.github.io/multi-value/js-api/index.html#exported-function-exotic-objects

Specifically, I paste some contents inline here:

To run a host function from the JavaScript object func, type functype, and list of WebAssembly values arguments, perform the following steps:

1. Let [parameters] → [results] be functype.
2. Let ret be ? Call(func, undefined, jsArguments).
3. If results is empty, return « ».
4. Otherwise, if results’s size is 1, return « ? ToWebAssemblyValue(ret, results[0]) ».
5. Otherwise:
   1. Let method be ? GetMethod(ret, @@iterator).
   2. If method is undefined, throw a TypeError.
   3. Let values be ? IterableToList(ret, method).
   4. Let wasmValues be a new, empty list.
   5. If values and results do not have the same length, throw a TypeError.
   6. For each value and resultType in values and results, paired linearly, append ToWebAssemblyValue(value, resultType) to wasmValues.
   7. Return wasmValues.

This patch implements calls from WebAssembly to JS, where the JS
function returns multiple values. The multiple values are returned as
an iterable.

Spec here:
https://webassembly.github.io/multi-value/js-api/index.html#exported-function-exotic-objects

Backed out changeset 5bd28470648f (Bug 1622828) for linux build bustages CLOSED TREE

https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&classifiedState=unclassified&fromchange=ebedd05e0d17283d20d2a4082c61cbc041765b54&selectedJob=295602693

https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=295602693&repo=autoland&lineNumber=7125

Backed out for build bustages.

Push with failure: https://treeherder.mozilla.org/#/jobs?repo=autoland&selectedJob=295618032&resultStatus=testfailed%2Cbusted%2Cexception&revision=5c02a3e6d56e78a513a67ad7c6119062c81a72a3

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=295618032&repo=autoland&lineNumber=4522

Backout: https://hg.mozilla.org/integration/autoland/rev/dc1c34d3f21b9a0ee12ac53c7db1a75804c21feb

https://hg.mozilla.org/mozilla-central/rev/4f66cfb5d2f2