Prevent arrow functions from being improperly inlined

[DavidANeil]

Closes #3756.

[blickly]

And thanks for the fix as well!

This looks good to me, so I'll work on getting it imported right away

[blickly]

This PR is now merged. Thanks!

One suggested improvement I heard during review was: only backing off for arrow functions that reference this/super/arguments rather than all arrow functions. This would still avoid the bug of #3756, while at the same time being able to inline arrow functions that don't reference those references from outer scopes (the more common case).

[DavidANeil]

My understanding was that isPropertyTree is a recursive function that only returns true if the lhs of the returned property access is this, so I feel like what you've suggested is already the case.

[blickly]

I think this might be best demonstrated with another unit test, similar to the one you created already, but checking that arrow functions can be inlined, something like:

    test(
        lines(
            "class Holder {",
            "    constructor() {",
            "        this.val = {internal: true};",
            "        this.context = {getVal: () => 5}",
            "    }",
            "}",
            "",
            "console.log(new Holder().context.getVal().internal);"),
        lines(
            "class Holder {",
            "    constructor() {",
            "        this.val = {internal: true};",
            "        this.context = {getVal: () => 5}",
            "    }",
            "}",
            "",
            "console.log(new Holder().context.val.internal);"));

If that unit test passes, that would prove that arrow functions can still be inlined by the pass.

[blickly]

I filed #3764 to track this work.