Adding tests for Array.prototype.copyWithin

[natashenka]

A security test for copyWithin. This checks that Array.prototype.copyWithin behaves correctly in the case that converting the start parameter to an integer has side effects. This caused a security bug in Moddable Moddable-OpenSource/moddable#123

[ljharb]

I tried adding these tests to the es6-shim; the "coerced-values-start-change-start" tests appear to fail in latest node (where the implementation is native), as well as with the es6-shim applied in both latest node and 0.10.

Specifically, it's getting an empty array where it expects a dense array of 24 undefineds - perhaps my test framework's assertion is incorrect (maybe it's different than compareArray), but can you confirm what the expected values should be for these tests?

[natashenka]

Yeah, I think this is a difference in how compare array works. The correct array to return is an empty array of length 20, but when it gets compared, the value is converted to undefined anyhow, but I'm guessing that doesn't happen in your harness. Anyhow, I updated it to compare the empty array, this makes it simpler, and hopefully will work with your harness as well.

[ljharb]

Tests pass on es6-shim on all envs as well as natively on v8.

[natashenka]

Just following up here. Is there anything I need to do to get this merged?