src/vs/workbench/api/common/extHostTask.ts/is
	export function is(value: tasks.ShellExecutionDTO | tasks.ProcessExecutionDTO | tasks.CustomExecutionDTO | undefined): value is tasks.CustomExecutionDTO {
		if (value) {
			let candidate = value as tasks.CustomExecutionDTO;
			return candidate && candidate.customExecution === 'customExecution';
		} else {
			return false;
		}
	}