Add Symbol.unscopables lexical scope test of compiled event handlers

[ExE-Boss]

Step 9 of get the current value of the event handler initialises the ObjectEnvironment records without setting the withEnvironment flag to true, which is required to make the ObjectEnvironment respect Symbol.unscopables.

As such, inline event handlers are required to include unscopable properties.

See also:

• ObjectEnvironment.HasBinding(n)
• WithStatement Evaluation
• Web Reality: The spec requires that unscopable properties be included in the compiled event handler lexical scope, but browsers omit them whatwg/html#6079
• Set withEnvironment of event handler scopes correctly whatwg/html#6086

[domenic]

All browsers fail this test, so it should probably be flipped, and the spec changed accordingly.

[annevk]

@ExE-Boss if there's a difference between setting withEnvironment on the innermost and most scopes it'd be good to have test coverage for that as well.

[ExE-Boss]

@annevk I’ve now also added test cases for the element and formOwner scopes.

[annevk]

This looks great. Just some minor tidying nits.

And I guess I should file a bug on Safari for not having the same unscopables. I wonder if there's a test for that somewhere... (Turns out this is tested by IDL harness tests and Safari has some failures for Fullscreen.)

[annevk]

(I decided to land this as everyone agrees this is the behavior these should have and we have a specification change pending that can land as soon as ECMAScript has had a slight refactoring.)