Class expression with extends clause is removed

[fasttime]

• Rollup Version: 1.13.1
• Operating System (or Browser):
• Node Version: 12.3.1

How Do We Reproduce?

Consider the following exported function containing an anonymous class expression.

export function isConstructor(fn) {
    if (fn === null)
        return false;
    try {
        void class extends fn { };
    } catch {
        return false;
    }
    return true;
}

In the bundled version, the class expression is dropped, causing the function to always branch out of the catch block.

function isConstructor(fn) {
    if (fn === null)
        return false;
    try {
    } catch {
        return false;
    }
    return true;
}

The only way I found to fix the issue is turning off treeshaking completely, which is not desirable.

Expected Behavior

isConstructor(true);   // false
isConstructor(isNaN);  // false
isConstructor(String); // true

Actual Behavior

isConstructor(true);   // true
isConstructor(isNaN);  // true
isConstructor(String); // true

[lukastaegert]

As a side-effect, #2892 should solve this issue as well without forcing a complicated deoptimization around super class definitions.