Check computed property names in strict mode

[bbrto21]

Escargot (please complete the following information):

• OS: Ubuntu 18.04
• Revision 6aa1ede

Describe the bug
Classes are always strict mode. Check computed property names and heritage expressions as well.

Test case

function test() {
    var b = class {
        [Object.preventExtensions({}).prop = 4]() {}
        constructor() { }
    };
}
try {
    test()
    throw "fail"
} catch (e) {
    print(e instanceof TypeError);
}

Expected behavior
true