Simplified custom DOMTokenList implementation.

TokenList.js

const forEach = Array.prototype.forEach;

module.exports = class TokenList {
	constructor(){
		this.tokens = {};
		this.add.apply(this, arguments);
	}

	add(){
		const { tokens } = this;
		forEach.call(arguments, token => {
			tokens[token] = true;
		});

		return this;
	}

	remove(){
		const { tokens } = this;
		forEach.call(arguments, token => {
			delete tokens[token];
		});

		return this;
	}

	contains(token){
		return Boolean(this.tokens[token]);
	}

	toggle(token, force){
		if(typeof force !== `undefined`){
			if(force){
				this.add(token);
				return true;
			}else{
				this.remove(token);
				return false;
			}
		}

		if(this.contains(token)){
			this.remove(token);
			return false;
		}

		this.add(token);
		return true;
	}

	get value(){
		return Object.keys(this.tokens).join(` `);
	}

	replace(oldToken, newToken){
		this.remove(oldToken);
		this.add(newToken);
	
		return this;
	}

	item(index){
		return Object.keys(this.tokens)[index];
	}
};

usage.js

const TokenList = require(`./TokenList`);

module.exports = function MyComponent(props){
	const classList = new TokenList(`class-a`);
	if(condition) classList.add(`class-b`);

	return <div class={classList.value} />;
}

Referring to your ideas, I have written a Classes to use it closer to the HTMLElement.classList.
https://gist.github.com/Mapaler/07b643000e5ef24535a1a860d957ed4d