initializedValue(lifetimeCounter) {
        if (this.lifetimeId !== lifetimeCounter) {
            this.ensureDeinitialized();
            this.mortalValue = this.initializer();
            this.lifetimeId = lifetimeCounter;
        }

        return this.mortalValue;
    }
Returns the mortal value stored in the slot, initializing or re-initializing it if necessary.
@param {!int} lifetimeCounter
@returns T