src/vs/workbench/contrib/tasks/common/problemCollectors.ts/WatchingProblemCollector/aboutToStart
class WatchingProblemCollector:	public aboutToStart(): void {
		for (let background of this.backgroundPatterns) {
			if (background.matcher.watching && background.matcher.watching.activeOnStart) {
				this._activeBackgroundMatchers.add(background.key);
				this._onDidStateChange.fire(ProblemCollectorEvent.create(ProblemCollectorEventKind.BackgroundProcessingBegins));
				this.recordResourcesToClean(background.matcher.owner);
			}
		}
	}