close(callback) {
		if (this.watching) {
			// When there is still an active watching, close this first
			this.watching.close(err => {
				this.close(callback);
			});
			return;
		}
		this.hooks.shutdown.callAsync(err => {
			if (err) return callback(err);
			// Get rid of reference to last compilation to avoid leaking memory
			// We can't run this._cleanupLastCompilation() as the Stats to this compilation
			// might be still in use. We try to get rid of the reference to the cache instead.
			this._lastCompilation = undefined;
			this._lastNormalModuleFactory = undefined;
			this.cache.shutdown(callback);
		});
	}
@param {RunCallback<void>} callback signals when the compiler closes
@returns {void}