src/vs/platform/contextkey/browser/contextKeyService.ts/ContextKeyService/createChildContext
class ContextKeyService:	public createChildContext(parentContextId: number = this._myContextId): number {
		if (this._isDisposed) {
			throw new Error(`ContextKeyService has been disposed`);
		}
		const id = (++this._lastContextId);
		this._contexts.set(id, new Context(id, this.getContextValuesContainer(parentContextId)));
		return id;
	}