Add method dynamically/Selective override prototype method in Flow

I have a constructor like this:

function IDBCrud(table: string): void {
   ...
}

IDBCrud.prototype.get = function(...) { ... }
IDBCrud.prototype.post = function(...) { ... }

And it using like this:

const accounts = new IDBCrud('Accounts');
accounts.get( ... );
accounts.create( ... );

But sometimes, I want to define method to object directly with same name as in property, so that invokes instead of prototype's method.

// Override get method for some reason
accounts.get = function( ... ) {
    // Do some stuffs...
    ...

    // Now call prototype get
    return this.__proto__.get.apply(this, arguments);
}

But when I ran flow, it fails with this:

 16: accounts.get = function(match, options) {
                  ^^^ property `get`. Property not found in
 16: accounts.get = function(match, options) {
     ^^^^^^^^^^^^ new object

Because IDBCrud doesn't have "get" property(or method). But if I just write them with empty value like this:

function IDBCrud(...): ... {
    this.get = function() {};
    this.create = function() {};
    ...
}

If should be work in that case, but if do that, I have to redefine every "get" method to invoke prototype's get method.

const accounts = new IDBCrud('accounts');
accounts.get = function() { ... };    // Override
accounts.get();    // works

const users = new IDBCrud('users');
users.get();    // Invokes users.get and it's empty function, instead of prototype.get

I don't wanna do that everytime I made IDBCrud instance, I just want to override it only it needed.

Without flow, it's not a problem, but with it, it fails.

So how do I achieve this with flow? Any advice will very appreciate it.