treeshake bug: __proto__ side effects in object literals ignored

[kzc]

• Rollup Version: rollup v2.50.0
• Operating System (or Browser): n/a
• Node Version (if applicable): v16.1.0

Repro

$ cat proto.js 
let getter_effect = "FAIL";
let setter_effect = "FAIL";
let proto = {
    get foo() {
        getter_effect = "PASS";
    },
    set bar(value) {
        setter_effect = "PASS";
    }
};
let obj1 = {
    __proto__: proto
};
let obj2 = {
    __proto__: proto
};
let unused = obj1.foo;
obj2.bar = 0;
console.log(getter_effect, setter_effect);

Expected Behavior

$ cat proto.js | node
PASS PASS

Actual Behavior

$ cat proto.js | rollup --silent | node
FAIL FAIL

$ cat proto.js | rollup --silent
let getter_effect = "FAIL";
let setter_effect = "FAIL";
console.log(getter_effect, setter_effect);

[lukastaegert]

Thanks for spotting, turns out there was quite some potential improvement left. Fix at #4112.