fix(template-compiler): inline iterator checks key is not index

[nolanlawson]

Details

Fixes #2702.

This is a fix for a somewhat orthogonal issue I discovered in #2699. I'm breaking it into its own PR to try to make things less confusing. (Hopefully?)

The compiler is supposed to throw an error if the key attribute is set to an index. But previously we weren't throwing the error for inline iterators:

<template>
  <div iterator:x={items} key={x.index}>{x.value}</div>
</template>

This PR fixes it so that it properly throws an error. I also added a bunch of tests to make sure that various flavors of iterators are working correctly (inline vs <template>, for:each vs iterator:*).

Does this pull request introduce a breaking change?

• ✅ No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• ⚠️ Yes, it does include an observable change.

If you try to set a key to an index on an inline iterator, it will now throw an error. I don't expect this to break anyone in practice, because in 234 and 236 we didn't support inline iterators at all – we introduced support for them (perhaps unintentionally) during development of 238.

GUS work item

W-10738523

[pmdartus]

Outside the broken test, this looks good to me.