Use initCustomEvent instead of new CustomEvent

[Rich-Harris]

This is one possible solution to #2018. It penalises non-IE11 users, which I don't like, but I'm not sure there's a clean alternative that doesn't involve polyfilling (which is a nuisance, even if the polyfill is small and simple).

Also, maybe it's better not to use legacy for this, since the other two places where it's used (for try-catching input.type = type, and replacing element.dataset.foo = bar with element.setAttribute('data-foo', bar)) are for IE9 (I think?), and it'd be nice if Svelte supported IE11 without the legacy flag since a lot of people are still in the unfortunate position of supporting it.

Any strong feelings either way?

[codecov-io]

Codecov Report

Merging #2101 into master will not change coverage.
The diff coverage is n/a.

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[Conduitry]

Yeah this seems reasonable. If the other legacy: true things are for even older versions of IE, it probably makes the most sense to just go with the IE11-compatible thing here. Requiring only transpiling and not polyfilling for IE11 sounds acceptable.

The cost thing this brings to even people not on IE11 is unfortunate, but slight. Even slighter if component events are less important in v3 than they were in v2.

[hontas]

What's unfortunate is that evt.initCustomEvent is not supported in Chrome versions prior to 59. Custom elements however are supported from Chrome 53. so this solution does not work for Chrome 53-59.

...yes I work on a project that has to support Chrome 53 😞 😢

To use new CustomEvent instead of document.createEvent('CustomEvent') and use the polyfill described here: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#Polyfill (247 bytes minified) would make more sense to me because because

1. it is polyfillable
2. it's not a deprecated api