I have narrowed the problem or bug to be triggered by either of the two commented lines being uncommitted. I was unable to find any similar questions but somebody else might have better search ability than I do.

Using the new runtime (Chrome V8) I am currently testing how to implement classes to an existing project but this testing is being done in it's own environment.

class test{

    //publicFoo = 321;   // <---- these two lines
    //#privateFoo = 456;

    constructor(){
        Logger.log("test created");
        this.foo = 123;   

    }

    getfoo(){
        return this.foo;
    }

    getPrivateFoo(){
        //return this.#privateFoo;
    }

}

function myClassTester(){
        const myTest = new test();

        Logger.log("myTest.foo: " + myTest.foo );
        Logger.log("myTest.getfoo(): " + myTest.getfoo() );
        Logger.log("myTest.publicFoo: " + myTest.publicFoo );
        Logger.log("myTest.privateFoo: " + myTest.privateFoo );
        Logger.log("myTest.getPrivateFoo(): " + myTest.getPrivateFoo() );

}

picture, no issues with the lines commented

picture, no functions to run when un commented

While verifying to write the question here I have also noticed if you do the edit online vs in clasp it does throw

Error: Line 3: Unexpected token = (line 2021, file "esprima.browser.js-bundle.js")

on saving...

Link to issue tracker ticket. https://issuetracker.google.com/150896358