constructor(_editor) {
        this._editor = _editor;
        this._instanceId = ++DynamicCssRules._idPool;
        this._counter = 0;
        this._rules = new Map();
        // We delay garbage collection so that hanging rules can be reused.
        this._garbageCollectionScheduler = new RunOnceScheduler(() => this.garbageCollect(), 1000);
    }
A helper to create dynamic css rules, bound to a class name.
Rules are reused.
Reference counting and delayed garbage collection ensure that no rules leak.